BANGKOK (AP) — A plaque honoring struggles for democracy in Thailand was removed from a royal field less than 24 hours after being installed by anti-government protesters and was submitted as evidence in connection with a complaint by officials that its installation was illegal, police said Monday.

The plaque had been installed Sunday at Sanam Luang, the historic field in the capital where tens of thousands of people rallied peacefully over the weekend. The two-day demonstration was the largest this year by protesters who are calling for new elections and reform of the monarchy.

An officer at a nearby police station said the government’s Fine Arts Department and the Bangkok city government filed a complaint that protesters had destroyed an archaeological site and “handed the plaque over to us to be used as evidence.”

“They are the responsible agencies over the area, as it is a public area registered as an archaeological site,” said the officer, police Supt. Worasak Pitsitbannakorn. “They reported the damage to us and we will have to determine which laws have been violated.” He said that the two agencies were responsible for removing the plaque, but that police were present as witnesses for legal reasons.

Sanam Luang is a designated royal site near Bangkok’s Grand Palace that until the last few years had been open to everyone. The plaque was installed in a part of the dirt field that was cemented over several years ago.

The plaque was a symbolic replacement for one mysteriously ripped out from the pavement and taken away three years ago. That plaque, located at a plaza in another part of Bangkok’s old center, honored the 1932 revolution that saw the military force a change from absolute monarchy to constitutional monarchy. It was replaced by a plaque praising the monarchy.

The new round brass plaque was installed Sunday by activists who made a hole in the Sanam Luang pavement and held a short religious ceremony.

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

Student protest leader Parit “Penguin” Chirawak, who was among those who installed the new plaque, said that its removal did not matter.

“What matters is this plaque, and its message has been installed in the people’s hearts,” Parit said Monday as he headed to the prosecutors’ office to deal with legal charges stemming from previous protests.

The protesters’ demands, including limiting the king’s powers, establishing tighter controls on palace finances and allowing open discussion of the monarchy, are controversial because the monarchy is considered sacrosanct in Thailand. The activists are considered especially bold because there is a harsh law mandating a three- to 15-year prison term for defaming the royal institution.

A representative of the protesters on Sunday delivered a petition addressed to the king seeking royal reforms. It was received by a police official, who promised to forward it to the Privy Council, the king’s advisers.

At the end of the weekend rally, Parit called for a general strike on Oct. 14, the anniversary of a 1973 student uprising that ended a military dictatorship after dozens were killed by soldiers and police.