By Panarat Thepgumpanat and Matthew Tostevin
BANGKOK (Reuters) - Thai authorities said on Monday they were considering charges against leaders of mass protests that called for reforms to the monarchy as police seized a symbolic plaque that declared Thailand belonged to the people and not the king.
Tens of thousands of people joined the protest at the weekend which cheered the demands to curb the powers of the monarchy of King Maha Vajiralongkorn - a revered institution according to the constitution.
"Leaders used inappropriate words about the institution and we will take full action against them," police spokesman Piya Uthayo told reporters, adding that 'lese majeste' charges against insulting the monarchy were a possibility.
Protesters have grown ever bolder during two months of demonstrations against the palace and military-dominated establishment. The Royal Palace did not respond to requests for comment on the protests or the protesters' demands.
Prime Minister Prayuth Chan-ocha, a former junta leader whom the protesters want removed from office, welcomed the fact the demonstration was peaceful. It was the biggest since he took power in a 2014 coup.
Police said the brass plaque, cemented in place near the Grand Palace in Bangkok after the protests, was being held as evidence.
Protest leader Parit "Penguin" Chiwarak expressed little surprise it had been removed. An electronic file of the plaque was shared online to enable people to make more of them and place them wherever they want.
"The plaque was already placed in the hearts of people. You may remove it but we will make a new one," he said.
Parit is among more than a dozen protest leaders who have already been arrested and bailed over earlier demonstrations -- though on other grounds than criticising the monarchy, Section 112 of the Thai criminal code.
Tul Sittisomwong, the leader of a royalist group, filed a complaint with police against Parit and two other protest leaders, saying they had violated the lese majeste law.
"Many Thais are not comfortable with people insulting his majesty," he told reporters.
Parit said on Twitter it was good that Tul had brought the complaint and would show whether the law was "still sacred". The prime minister said in June that lese majeste was not being applied in line with the king's wishes.
Another protest leader, Panusaya Sithijirawattanakul, told Reuters they had not insulted the monarchy.
“We don't want to topple the institution. Our proposal is reform, not revolution," she said.
The hashtag that translates as #Cancel112 was among the top trending on Twitter in Thailand - with more than 256,000 tweets.
The demonstrators say the constitution gives the king too much power and was engineered to let Prayuth keep power after elections last year. He says that vote was fair.
The symbolism of the plaque is its resemblance to 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.
"Everyone knew it would disappear soon but the success of creating it is something that will continue. It is still an important symbol," said barber Craig Kunakorn, 33, who visited the spot where the new plaque had been removed.
(Additional reporting by Orathai Sriring, Juarawee Kittisilpa; Writing by Ed Davies; Editing by Matthew Tostevin, Shri Navaratnam and Alison Williams)