Anti-government demonstrators occupying a historic field in the Thai capital have installed a plaque symbolising the country’s transition to democracy.

The plaque replaces the original that was mysteriously stolen three years ago.

The mass student-led rally that began on Saturday was the largest in a series of protests this year, with thousands camping overnight at Sanam Luang field near the royal palace. Protesters are calling for new elections and reform of the monarchy.

A group of activists drilled a hole in front of a makeshift stage and, after Buddhist rituals, laid down a round brass plaque in cement to commemorate a 1932 revolution that changed Thailand from an absolute monarchy to a constitutional monarchy.

The inscription reads: “At the dawn of September 20, here is where the people proclaim that this country belongs to the people.”

In April 2017, the original plaque vanished from Bangkok’s Royal Plaza and was replaced by one praising the monarchy.

Student leader Parit “Penguin” Chiwarak told the crowd: “The nation does not belong to only one person, but belongs to us all. Therefore, I would like to ask holy spirits to stay with us and bless the people’s victory.”

Pro-democracy protesters hold up three fingers, the movement’s symbol of resistance salute, as they march close to Sanam Luang field in Bangko
Pro-democracy protesters hold up three fingers, the movement’s symbol of resistance salute, as they march close to Sanam Luang field in Bangkok (AP)

Another activist, Panusaya Sithijirawattanakul, said their demands do not propose getting rid of the monarchy, adding: “They are proposals with good intentions to make the institution of the monarchy remain graciously above the people under democratic rule.”

Protesters are calling for limits on the king’s powers, tighter controls on palace finances and open discussion of the monarchy. Their boldness was 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.

The protesters later attempted to march towards the Grand Palace to hand over a petition seeking royal reforms to the head of the Privy Council, the king’s advisers, but were blocked by a police barricade. One demonstrator was allowed to deliver the petition, addressed to the king. It was received by a police official, who promised to forward it to the council.

Police officers guard the area surrounding the Grand Palace
Police officers guard the area surrounding the Grand Palace (Wason Wanichakorn/AP)

Just before the rally ended, Mr Parit called for a general strike on October 14, the anniversary of a popular student uprising in 1973 that ended the military dictatorship after dozens were killed by police. He also called for another protest on Thursday outside parliament to follow up on their demands.

Organisers had predicted that as many as 50,000 people would take part in the weekend’s protest, but Associated Press reporters estimated that around 20,000 people were present by Saturday evening.