The plaque said the "country belongs to the people — not to the king." Protests against the monarchy have been gathering momentum since July.
Pro-democracy demonstrators placed a plaque in the ground next to Thailand's Grand Palace on Sunday, in a statement to the royalty that the country belonged to the people.
According to Bangkok news site Khaosod, the plaque reads "The people let it be known that our country belongs to the people – not to the king, as has been deceitfully claimed," which references the 1932 revolution that changed the country from an absolute monarchy to a constitutional monarchy.
The news site said the plaque was put in the ground of the Sanam Luang field at about 6:39 a.m. local time (2339 Saturday UTC).
Protests have been growing in the Southeast Asian country since July, breaking a taboo of criticizing the monarchy and attempting to oust the government and bring in a new constitution and elections.
Protesters' demands include limiting the king's powers, tighter controls on palace finances and open discussion of the monarchy.
Thai law mandates a 3-to-15 year prison sentence for defaming the monarchy.
Government spokesman Anucha Burapachaisri said police would not use violence against the demonstrators and that it was up to them to prosecute any illegal speech. Police have held back from protests and have not intervened.
Many protesters slept in the field overnight, waking on Sunday morning to take part in one of the largest demonstrations in the country in years. Tens of thousands cheered for reforms to the Thai monarchy and called for the removal of Prime Minister Prayuth Chan-ocha, a former junta leader.
"The nation belongs to no one person but to all of us," said Parit "Penguin" Chiwark, one of the protest leaders, at the ceremony for installing the plaque. "Down with feudalism, long live the people."
Protesters initially said they would march from the scene of the ceremony at 8 a.m. local time Sunday. But Khaosod reported that they would instead submit a petition to King Maha Vajiralongkorn. The website added that security forces placed barbed wire around the Government House ahead of the anticipated march.
The protesters handed a set of demands to the Royal Guard police after their march was blocked.
Panusaya Sithijirawattanakul, one of the protest leaders, said the police told her that the letter would be sent to police headquarters.
Protesters said they will converge in front of the parliament building on Thursday for the next round of protests.
kbd/sms (AP, Reuters)