BANGKOK • A plaque commemorating a 1932 coup in Thailand that saw absolute monarchy abolished and democracy established has gone missing, police in Bangkok said yesterday, prompting outcry from pro-democracy activists.

The 1932 coup, also known as the Siamese Revolution, was a crucial turning point in Thai history and ended nearly seven centuries of absolute monarchy, paving the way for political and social reforms.

Since then, Thailand has gone through a shaky experiment with democracy and has witnessed a succession of political protests and coups.

Thailand has been governed by a junta since the latest coup in 2014, which saw the military overthrow a democratically elected government.

The plaque, which was embedded in a square in central Bangkok, was removed and replaced with a new one which highlights the importance of the monarchy. "It is good to worship the Buddhist trinity, the state, one's own family, and to be faithful to one's monarch and allow oneself to be the engine that brings prosperity to the state," the new plaque reads.

Police in the Dusit district where the plaque was located said they were not sure who removed it and were investigating.

Ultra-royalist groups had previously threatened to remove the plaque.

Activists said that the plaque's removal was a bid by royalist conservatives to rewrite history.

"This is another attempt to alter the history of democracy in this country," Mr Than Rittiphan, a member of the student-led New Democracy Movement which has protested against military rule, told Reuters. "It is nothing more than fascist rhetoric aimed at brainwashing the next generation," he said.

The government, led by Prime Minister Prayut Chan-o-cha, a former army chief and staunch royalist, has stepped up prosecution of critics of the monarchy under a harsh royal insult law.

REUTERS