BANGKOK (AP) — A plaque honoring struggles for democracy in Thailand has been removed less than 24 hours after it was installed by anti-government protesters at a historic royal field in the capital.

The plaque had been installed Sunday at Sanam Luang, the field 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.

On Monday morning, journalists saw hardened cement in place of the plaque. No one has taken responsibility for its removal. Taiwuth Kankaew, who as public works director for Bangkok's city government is responsible for surveying and repairing damages in Sanam Luang, said he had no knowledge of who removed it.

The plaque at Sanam Luang 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 drilled 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 Monday that its removal did not matter.

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

Sanam Luang is a designated royal site that until the last few years was open to everyone. It is near Bangkok's Grand Palace.

On Monday, a representative of the government’s Fine Arts Department filed a complaint with police charging that protesters had destroyed an archaeological site.

The protesters’ demands include limiting the king’s powers, establishing tighter controls on palace finances and allowing open discussion of the monarchy. Their boldness is unprecedented, as the monarchy is considered sacrosanct in Thailand, with a harsh law that mandates a three- to 15-year prison term for defaming it.

Also Sunday, a representative of the protesters 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 student uprising in 1973 that ended a military dictatorship after dozens were killed by police.