(Adds witness, details on protest)
By Panarat Thepgumpanat and Matthew Tostevin
BANGKOK, Sept 21 (Reuters) - A plaque placed by Thai pro-democracy protesters near the Grand Palace in Bangkok that declared that Thailand belongs to the people and not the king has been removed, as police warned on Monday they may charge those behind the symbolic gesture.
The plaque was placed on Sunday after a rally by tens of thousands of people who cheered calls for reforms to the monarchy of King Maha Vajiralongkorn.
"I've received a report that the plaque is gone but I don't know how and I don't know who did it," Bangkok's deputy police chief Piya Tawichai told Reuters.
"Police are checking with the BMA (Bangkok Metropolitan Administration) and checking who took it out as the plaque is part of the evidence to charge the protest group (for this wrongdoing)," Piya said.
At the biggest demonstration in Thailand in years, protesters cheered calls for reform of the monarchy as well as for the removal of Prime Minister Prayuth Chan-ocha, a former junta leader, and a new constitution and elections.
After the protest, people queued up to take pictures next to the plaque, which also features a hand giving the three-finger salute adopted by pro-democracy protesters.
But far from all Thais support the new plaque, which resembles one that had commemorated the end of absolute monarchy in 1932 and which was removed from outside a royal palace in 2017, after Vajiralongkorn took the throne.
Prominent right-wing politician Warong Dechgitvigrom said on Sunday the plaque was inappropriate and the king was above politics.
"It didn't achieve anything," he told Reuters. "These actions are symbolically against the king, but the king is not an opponent."
Craig Kunakorn, 33, a barber who had been at the protests came to visit the spot where the plaque had been cemented into the ground on Monday.
"Everyone knew it would disappear soon but the success of creating it is something that will continue. It is still an important symbol," Craig said.
Anon Nampa, an activist and human rights lawyer, told Reuters the plaque should be returned to the people.
"We will go and file a complaint to police today to find that plaque which is the people's property and who took it," said Anon, one of the leading figures in the loosely organised protest movement.
Protesters have grown ever bolder during two months of demonstrations against Thailand's palace and military-dominated establishment, breaking a longstanding taboo on criticising the monarchy - which is illegal under lese majeste laws.
The demonstrators say the constitution gives the king too much power and that it was engineered to allow Prayuth to keep power after elections last year. He says that vote was fair. The next protest is scheduled for Thursday. Protest leaders also called on Thais to take Oct. 14 off work to show their support for change.
(Writing by Ed Davies; Editing by Stephen Coates)