A plaque symbolizing Thailand's transition to democracy has been removed less than 24 hours after it was installed by anti-government demonstrators in a historic royal field.

The weekend demonstrations were the largest this year by protesters who vowed to press on with calls for new elections and reform of the monarchy. The plaque had been installed at Sanam Luang on Sunday to replace one mysteriously ripped out and stolen three years ago.

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Pro-democracy student leaders hold a plaque declaring "This country belongs to the people" at the Sanam Luang field during a protest in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoSakchai Lalit)

A plaque symbolizing Thailand's transition to democracy has been removed less than 24 hours after it was installed by anti-government demonstrators in a historic royal field.

Pro-democracy student leaders hold a plaque declaring "This country belongs to the people" at the Sanam Luang field during a protest in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoSakchai Lalit)

Sanam Luang is a designated royal site, which until the last few years was open to all. It is located near the Grand Palace.

Pro-democracy student leaders install a plaque declaring "This country belongs to the people" at the Sanam Luang field during a protest in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoSakchai Lalit)

“At the dawn of Sept. 20, here is where the people proclaim that this country belongs to the people,” read part of the inscription.

Pro-democracy protesters raise a three-fingers, symbol of resistance salute as others observe Buddhist religious rituals during a rally at Sanam Luang in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators who occupied a historic field in Thailand's capital overnight continued with their rally on Sunday to support the demands of a student-led protest movement for new elections and reform of the monarchy. (AP PhotoGemunu Amarasinghe)

Pro-democracy protesters raise a three-fingers, symbol of resistance salute as others observe Buddhist religious rituals during a rally at Sanam Luang in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators who occupied a historic field in Thailand's capital overnight continued with their rally on Sunday to support the demands of a student-led protest movement for new elections and reform of the monarchy. (AP PhotoGemunu Amarasinghe)

A pro-democracy protester raises a three-fingers, symbol of resistance salute during a rally at Sanam Luang in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators who occupied a historic field in Thailand's capital overnight continued with their rally on Sunday to support the demands of a student-led protest movement for new elections and reform of the monarchy. (AP PhotoGemunu Amarasinghe)

A pro-democracy protester raises a three-fingers, symbol of resistance salute during a rally at Sanam Luang in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators who occupied a historic field in Thailand's capital overnight continued with their rally on Sunday to support the demands of a student-led protest movement for new elections and reform of the monarchy. (AP PhotoGemunu Amarasinghe)

Pro-democracy protesters gather at the Sanam Luang field during a protest in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoWason Wanichakorn)

Pro-democracy protesters gather at the Sanam Luang field during a protest in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoWason Wanichakorn)

Pro-democracy protesters wave the national flag at the Sanam Luang field during a protest in Bangkok, Thailand, Saturday, Sept. 19, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoWason Wanichakorn)

Pro-democracy protesters wave the national flag at the Sanam Luang field during a protest in Bangkok, Thailand, Saturday, Sept. 19, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoWason Wanichakorn)

On Monday morning, journalists who went to Sanam Luang saw hardened cement in place of the plaque. A couple of police officers arrived moments later. It was unclear who ordered the removal of the plaque, and when it was removed.

Pro-democracy student leaders hold a plaque declaring "This country belongs to the people" at the Sanam Luang field during a protest in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoSakchai Lalit)

Pro-democracy student leaders hold a plaque declaring "This country belongs to the people" at the Sanam Luang field during a protest in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoSakchai Lalit)

Sanam Luang is a designated royal site, which until the last few years was open to all. It is located near the Grand Palace.

Taiwuth Kankaew, the director of the Department of Public Works of Bangkok Metropolitan Authority, responsible for surveying and repairing damages in Sanam Luang. said he has no knowledge who removed the plaque.

On Sunday, a group of activists had drilled a hole in front of a makeshift stage and, after Buddhist rituals, laid a round brass plaque in cement to commemorate the 1932 revolution that changed Thailand from an absolute monarchy to a constitutional monarchy.

Pro-democracy student leaders hold a plaque declaring "This country belongs to the people" at the Sanam Luang field during a protest in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoSakchai Lalit)

Pro-democracy student leaders hold a plaque declaring "This country belongs to the people" at the Sanam Luang field during a protest in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoSakchai Lalit)

“At the dawn of Sept. 20, here is where the people proclaim that this country belongs to the people,” read part of the inscription.

Pro-democracy student leaders install a plaque declaring "This country belongs to the people" at the Sanam Luang field during a protest in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoSakchai Lalit)

Pro-democracy student leaders install a plaque declaring "This country belongs to the people" at the Sanam Luang field during a protest in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoSakchai Lalit)

Pro-democracy protesters raise a three-fingers, symbol of resistance salute as others observe Buddhist religious rituals during a rally at Sanam Luang in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators who occupied a historic field in Thailand's capital overnight continued with their rally on Sunday to support the demands of a student-led protest movement for new elections and reform of the monarchy. (AP PhotoGemunu Amarasinghe)

Pro-democracy protesters raise a three-fingers, symbol of resistance salute as others observe Buddhist religious rituals during a rally at Sanam Luang in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators who occupied a historic field in Thailand's capital overnight continued with their rally on Sunday to support the demands of a student-led protest movement for new elections and reform of the monarchy. (AP PhotoGemunu Amarasinghe)

A pro-democracy protester raises a three-fingers, symbol of resistance salute during a rally at Sanam Luang in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators who occupied a historic field in Thailand's capital overnight continued with their rally on Sunday to support the demands of a student-led protest movement for new elections and reform of the monarchy. (AP PhotoGemunu Amarasinghe)

A pro-democracy protester raises a three-fingers, symbol of resistance salute during a rally at Sanam Luang in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators who occupied a historic field in Thailand's capital overnight continued with their rally on Sunday to support the demands of a student-led protest movement for new elections and reform of the monarchy. (AP PhotoGemunu Amarasinghe)

Pro-democracy protesters gather at the Sanam Luang field during a protest in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoWason Wanichakorn)

Pro-democracy protesters gather at the Sanam Luang field during a protest in Bangkok, Thailand, Sunday, Sept. 20, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoWason Wanichakorn)

Pro-democracy protesters wave the national flag at the Sanam Luang field during a protest in Bangkok, Thailand, Saturday, Sept. 19, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoWason Wanichakorn)

Pro-democracy protesters wave the national flag at the Sanam Luang field during a protest in Bangkok, Thailand, Saturday, Sept. 19, 2020. Thousands of demonstrators turned out Saturday for a rally to support the student-led protest movement's demands for new elections and reform of the monarchy. (AP PhotoWason Wanichakorn)