Heavy police presence in Athens has prevented an escalation of clashes with anarchists and other extreme leftists marking the 46th anniversary of a student uprising against the then-ruling Greek military dictatorship.

Over 20,000 people made the traditional march from the National Technical University of Athens, site of the 1973 uprising, to the U.S. Embassy on Sunday.

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University students hold a blood-stained Greek flag from the deadly 1973 student uprising during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Heavy police presence in Athens has prevented an escalation of clashes with anarchists and other extreme leftists marking the 46th anniversary of a student uprising against the then-ruling Greek military dictatorship.

University students shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

Police say they arrested 28 people in clashes after the march, mostly in the Exarchia neighborhood near the university, and detained a further 13.

Protesters shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

Protesters shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

University students shout slogans as they hold a blood-stained Greek flag from the deadly 1973 student uprising during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

University students shout slogans as they hold a blood-stained Greek flag from the deadly 1973 student uprising during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

University students hold a blood-stained Greek flag from the deadly 1973 student uprising during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

University students hold a blood-stained Greek flag from the deadly 1973 student uprising during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

Protesters shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

Protesters shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

University students shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

University students shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

University students shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

University students shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Riot police guard at Syntagma square during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

Riot police guard at Syntagma square during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

Riot police guard the U.S. Embassy during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Riot police guard the U.S. Embassy during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Former Greek Prime Minister Alexis Tsipras, center, shouts slogans as he heads the block of his leftish Syriza party during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (Panayiotis TzamarosInTime News via AP)

Former Greek Prime Minister Alexis Tsipras, center, shouts slogans as he heads the block of his leftish Syriza party during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (Panayiotis TzamarosInTime News via AP)

Riot police guard the U.S. Embassy as protesters take part in a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Riot police guard the U.S. Embassy as protesters take part in a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Riot police guard the U.S. Embassy as protesters take part in a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Riot police guard the U.S. Embassy as protesters take part in a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Riot police guard the U.S. Embassy as protesters take part in a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Riot police guard the U.S. Embassy as protesters take part in a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Members of the Greek Communist Party shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

Members of the Greek Communist Party shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

A protester burns a US flag during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

A protester burns a US flag during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

A protester burns a US and NATO flags during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

A protester burns a US and NATO flags during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

Police say about 10,000 people participated in a Communist Party rally and a further 1,000 marched with the formerly ruling leftist Syriza party, both separate from the main march with over 10,000 participants. Former Prime Minister Alexis Tsipras started the Syriza march but left before the halfway point.

University students hold a blood-stained Greek flag from the deadly 1973 student uprising during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

University students hold a blood-stained Greek flag from the deadly 1973 student uprising during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Police say they arrested 28 people in clashes after the march, mostly in the Exarchia neighborhood near the university, and detained a further 13.

University students shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

University students shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

Protesters shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

Protesters shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

University students shout slogans as they hold a blood-stained Greek flag from the deadly 1973 student uprising during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

University students shout slogans as they hold a blood-stained Greek flag from the deadly 1973 student uprising during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

University students hold a blood-stained Greek flag from the deadly 1973 student uprising during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

University students hold a blood-stained Greek flag from the deadly 1973 student uprising during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

Protesters shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

Protesters shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

University students shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

University students shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

University students shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

University students shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Riot police guard at Syntagma square during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

Riot police guard at Syntagma square during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

Riot police guard the U.S. Embassy during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Riot police guard the U.S. Embassy during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Former Greek Prime Minister Alexis Tsipras, center, shouts slogans as he heads the block of his leftish Syriza party during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (Panayiotis TzamarosInTime News via AP)

Former Greek Prime Minister Alexis Tsipras, center, shouts slogans as he heads the block of his leftish Syriza party during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (Panayiotis TzamarosInTime News via AP)

Riot police guard the U.S. Embassy as protesters take part in a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Riot police guard the U.S. Embassy as protesters take part in a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Riot police guard the U.S. Embassy as protesters take part in a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Riot police guard the U.S. Embassy as protesters take part in a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Riot police guard the U.S. Embassy as protesters take part in a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Riot police guard the U.S. Embassy as protesters take part in a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoPetros Giannakouris)

Members of the Greek Communist Party shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

Members of the Greek Communist Party shout slogans during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

A protester burns a US flag during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

A protester burns a US flag during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

A protester burns a US and NATO flags during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)

A protester burns a US and NATO flags during a rally in Athens, Sunday, Nov. 17, 2019. Several thousands people march to the U.S. Embassy in Athens under tight police security to commemorate a 1973 student uprising that was crushed by Greece's military junta, that ruled the country from 1967-74. (AP PhotoYorgos Karahalis)