Bangkok: Thailand's military government has banned public discussion on the mysterious disappearance of a plaque commemorating a revolution that established democratic rule in the country 85 years ago.

The Foreign Correspondent's Club of Thailand accepted a police request to cancel a panel discussion on the disappearance on Wednesday night, saying in a statement it "had no choice but to comply".

The small bronze plaque in the Royal Plaza commemorated Thailand's 1932 revolution. Credit:AP

A letter from police said the event "posed a threat to national security".

The whereabouts of the 30-centimetre plaque that disappeared in April from a pavement near Bangkok's Royal Plaza, an area in the midst of government buildings and military installations, has captivated Thailand.