A Thai court has sentenced the leader of the opposition Red Shirt movement to two years in prison for defaming a

former prime minister, a move analysts said was the latest attack against critics of the military government.

Wednesday's ruling comes days after the retroactive impeachment and announcement of corruption charges against ex-prime minister Yingluck Shinawatra, whose toppled government was backed by the Red Shirts before May's army coup.

The court convicted Red Shirts chairman Jatuporn Prompan on two counts of defamation against former royalist prime minister Abhisit Vejjajiva during speeches he made in October 2009.

"The verdict is two years in prison without suspension," a court official told AFP news agency.

Jatuporn's lawyer, Winyat Chatmontree, confirmed the sentencing, saying his client would not be imprisoned until the conclusion of his appeal.

"The court has approved bail. After this we will submit an appeal," AFP quoted Chatmontree as saying.

Anti-coup voices, including lawmakers from Yingluck's former ruling Puea Thai party, academics and students' groups, have been silenced after the army outlawed political gatherings, briefly summoned dissenters and censored the media.

The once mighty Red Shirts have gone to ground with Jatuporn one of their few public faces, back on the small screen after the military government lifted a ban on political television channels, despite his every word coming under close scrutiny by authorities.

The leader has swapped his once rabble-rousing rhetoric for cautionary language, urging followers against taking to the streets after last Friday's decision against Yingluck.