Demonstrators clashed with riot police in central Athens Thursday during the first of a series of events to mark the 10th anniversary of the fatal police shooting of a teenager that sparked Greece's most extensive riots in decades.

Hooded youths threw rocks and other objects at riot police shortly after the start of a student march, and police responded with tear gas. At least one detention was reported.

Authorities deployed about 2,500 police and a helicopter for marches planned to remember Alexis Grigoropoulos, a 15-year-old who died on a night out with friends in the Exarchia district of the capital after being shot by a policeman following an altercation on Dec. 6, 2008.

Riot police detain a protester in Athens, on Thursday, Dec. 6, 2018 during a rally commemorating the killing of a 15-year old student back in 2008. Hundreds of school and university students march through the streets of the Greek capital to mark the tenth anniversary of a fatal police shooting of a teenager in Athens that sparked the worst rioting Greece had seen in decades. (AP PhotoPetros Giannakouris)

Riot police detain a protester in Athens, on Thursday, Dec. 6, 2018 during a rally commemorating the killing of a 15-year old student back in 2008. Hundreds of school and university students march through the streets of the Greek capital to mark the tenth anniversary of a fatal police shooting of a teenager in Athens that sparked the worst rioting Greece had seen in decades. (AP PhotoPetros Giannakouris)

The teenager's killing sparked major riots which spread to cities across the country and lasted for weeks, with rioters smashing store windows, looting and setting cars alight. The policeman was convicted of murder and sentenced to life in prison. The second policeman present was sentenced to 10 years and was granted conditional release several years ago.

Annual marches marking the anniversary often turn violent, with clashes breaking out between police and protesters.

On Thursday, a second march was scheduled for later in the day, while similar demonstrations were being held in the northern city of Thessaloniki. A gathering was to be held Thursday night at the spot where Grigoropoulos died.