0829 GMT March 21, 2018
Authorities say they have no idea what happened to the memorial in the capital Bangkok which marked the spot in 1932 where a group of revolutionaries declared an end to absolute powers of the then monarch Rama VII, AFP reported.
The date was pivotal in Thai history, leading to the country's first constitution among other political reforms. The plaque was reported missing last Friday.
In more recent times the memorial has become a touchpaper for Thailand's current political divide between democrats and monarchial conservatives – some of whom had called for the plaque to be ripped up.
Now the disappearance of the small but symbolic memorial has rekindled fears among democracy campaigners that ascendant arch-royalists are trying to rewrite Thai history.
Official denials over the plaque's whereabouts have stretched credulity given the circular bronze memorial was embedded in the road of the well-policed Royal Plaza.
It was also replaced by a new plaque calling on Thais to be loyal to Buddhism, the state, family and the monarchy – core values of conservatives.
"I don't want this to become an issue," Junta chief Prayut Chan-o-Cha, an ultra-monarchist who seized power three years ago, told reporters.
Warning democracy activists from ramping up the issue he said it was "useless to ask for its return".
His comments came as soldiers detained a prominent activist as he tried to file a complaint with the government over the missing plaque.
There was a heavy police presence around the replacement memorial and officers refused to let an AFP journalist photograph it.
After days of obfuscation by police, Prayut on Tuesday said he had ordered an investigation into the issue.
Small groups of protesters have staged demonstrations or taken to social media in recent days to voice outrage over the plaque's removal.
Earlier this month Thailand's new monarch King Maha Vajiralongkorn, or Rama X, requested a series of amendments to the kingdom's charter that analysts said would boost his constitutional powers.
The New Democracy Movement, an anti-junta student group, said the disappearance of the plaque "showed a lack of respect for the past" and to those who brought democracy to Thailand.