BANGKOK • Thai authorities have detained an activist attempting to mark the 1932 revolution that ended absolute monarchy, police said, a historical event that has become increasingly taboo under an ultra-royalist junta.

The bloodless revolution on June 24, 1932 launched parliamentary democracy in Thailand, but the decades since have been punctuated by frequent military coups, including the putsch that ushered in the current army-led government.

In recent years, small groups of democracy activists have gathered each anniversary to lay flowers on a small bronze plaque marking the spot where revolutionaries proclaimed the end of absolute monarchy in a fiery speech lambasting the king.

But the 76cm plaque was mysteriously removed from Bangkok's Royal Plaza this April and replaced with a new marker bearing a royalist message. The authorities have not offered an explanation for the incident, and earlier this month, the police banned people from gathering around the memorial on the revolution's anniversary.

Yesterday, activist Ekachai Hongkangwan was taken to a Bangkok military base for "attitude adjustment" after he tried to lay a replica of the original plaque on top of the new one, said deputy Metropolitan Police commissioner Phanurat Lukboon.

"He is now being held at the 11th army circle," the officer said, referring to an army barracks.

The drama around the democracy plaque comes as royalist voices are ascendant in Thailand, which is under the grip of a junta that grabbed power in 2014.

Use of the draconian royal defamation law has surged under its rule, with offenders handed decades-long prison sentences for criticism that is often posted on social media.

Earlier this month, a Thai man was sentenced to 35 years in prison for Facebook posts deemed insulting to the royal family.

AGENCE FRANCE-PRESSE