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.
On Monday morning, journalists who went to Sanam Luang saw hardened cement in place of the plaque. Police officers arrived moments later. It was unclear who ordered the removal of the plaque, and when it was removed.
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 had 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.
“At the dawn of Sept. 20, here is where the people proclaim that this country belongs to the people,” read part of the inscription.
