Prime Minister Prayut Chan-o-cha enters Tuesday's cabinet meeting with a warning and his opinion: 'Old subjects are just history.' (Reuters photo)

Prime Minister Prayut Chan-o-cha has ordered a probe into the disappearance of the 1932 Siamese Revolution memorial plaque and warned against any attempts to exploit the issue to stir up conflict.

Speaking after Tuesday's cabinet meeting, Gen Prayut said he has instructed security officers to investigate the matter, and added that it was not a matter of life or death and no groups should use it to re-ignite conflict while the country is returning to democracy.

"Democracy was introduced to the country more than 80 years ago and democracy depends on all Thais, not the plaque," Gen Prayut said.

"Some groups are trying to call for the plaque's return, but I think this is useless. If you want the country to return to democracy quickly, you'd better work together so the country can move forward. I don't want any protests to take place again," the prime minister said, and the government will take action against anyone who breaks security laws.

He added: "Would it be better for us to look ahead at the future? Old subjects are just history."

Deputy Prime Minister and Defence Minister Prawit Wongsuwon said he knew nothing about the disappearance of the plaque.

Gen Prawit said he did not follow up on the disappearance because it was not a matter of grave concern. "It is not a bread-and-butter concern. Just let the authorities do their job," Gen Prawit said.

On Sunday, deputy national police chief Srivara Ransibrahmanakul said the disappearance of the plaque cannot be considered a case of theft unless the owner shows up to file a complaint.

Anyone who claims to be its owner must come up with proof of ownership so police can proceed with the case, Pol Gen Srivara said.

He also said police had asked several agencies such as the Dusit district office and the Fine Arts Department, and they had confirmed the plaque did not belong to them.

In light of this, the plaque is not a public asset nor an ancient object, Pol Gen Srivara said.

"Since it is not clear who owns the plaque, how can police proceed with a theft case?" he said, adding that no wrongdoing had yet been committed so police had no authority to take legal action against anyone.

On Monday, Prit Rattanakul Serireungriddhi, a 30-year-old grandson of Luang Serireungriddhi, who was a member of the Khana Ratsadon group, filed a complaint at Dusit police station asking them to find the plaque, which mysteriously disappeared from the Royal Plaza and has been replaced by a different one.

Khana Ratsadon was a group of military and civil officers that staged the 1932 revolution, changing the country's absolute monarchy to a constitutional one.

Bangkok governor Aswin Kwanmuang said he had been unaware the plaque had gone missing as he had been busy looking after people during Songkran in Bangkok.

Pol Gen Aswin said the plaque was not the responsibility of the Bangkok Metropolitan Administration. The spot where the plaque lay is not a public place, he said, without elaborating.

"When something is missing, you should alert the police, not the BMA," the Bangkok governor added.

Political activist Srisuwan Janya, secretary-general of the Association for the Protection of the Constitution, was whisked away in a military van Tuesday morning for talks at the 11th Military Circle in Bangkok's Dusit district, according to media reports.

It happened after Mr Srisuwan showed up at the Government House public service centre to submit a letter to Gen Prayut asking him to look for the plaque.

On his arrival at the centre, Mr Srisuwan was approached by soldiers and police. He was then ordered to board a van waiting nearby. Two men were initially allowed to accompany him.

On the way to the military base , the two men were told to get out of the vehicle. They were each given 100 baht to cover their transport costs, they said.

One of them said Mr Srisuwan refrained from speaking but looked worried.

Before Mr Srisuwan's arrival at Government House, a group of people claiming to belong to the Group of Sovereign Thai People for the Nation, Religion and Monarchy submitted a letter to Gen Prayut, calling for him to remove the new plaque and place it somewhere else.

The brass plaque, fitted into the road surface near the King Rama V statue, marks the spot where Khana Ratsadon announced the revolution in 1932.

The inscription reads: "Here at dawn on June 24, 1932, Khana Ratsadon brings into being the constitution for the sake of the country's prosperity."

The original plaque has now been replaced with a new plaque which bears a distinctly different message: "May Siam be blessed with prosperity forever. May the people be happy and cheerful and become the strength of the country.

The rim around the surface of the new plaque also bears another inscription reading: "The respect for Phra Ratanattaya [The Three Jewels -- Buddha, Dhamma, Sangha], the state, one's family, and the faithfulness towards one's King will all contribute to the prosperity of one's state."