BANGKOK (Reuters) - Thailand’s Supreme Court jailed a firebrand opposition leader for a year on Thursday for defaming a former prime minister, overturning two previous court rulings, in 2012 and 2014, that dismissed the charge against him.

Jatuporn Prompan, a well-known 'red shirt' leader, gestures during a news conference at his encampment in Nakhon Pathom province on the outskirts of Bangkok May 22, 2014. REUTERS/Chaiwat Subprasom/File Photo

Jatuporn Prompan is a key leader of the “red shirt” supporters of former Prime Minister Thaksin Shinawatra and his sister, Yingluck Shinawatra, who also served as prime minister. Their governments were both ousted in military coups, in 2006 and 2014 respectively.

Jatuporn led street protests in 2010 when the supporters of the populist former telecommunications tycoon Thaksin occupied a Bangkok shopping district for nearly two months, calling for the then prime minister, Abhisit Vejjajiva, to resign.

The Supreme Court found Jatuporn guilty of calling Abhisit a murderer in a speech during the protests, which ended in bloody confrontation between the military and demonstrators in which more than 90 people were killed, most of them civilians.

Jatuporn’s speech had not been fact-checked and had been made for political gain, causing damage to Abhisit’s reputation, the court said.

Jatuporn did not speak to the court or to reporters, but only smiled as he was led away to jail, media reported.

Confrontation between the Shinawatras and their legions of supporters, especially among the rural poor, on the one hand, and the military-dominated establishment, based in Bangkok, on the other, has divided Thailand for more than a decade.

Jatuporn won a seat in parliament in a 2011 general election, when Yingluck led the Shinawatras’ party to victory. Thaksin has lived in self-exile since 2008 to avoid a corruption conviction he says was politically motivated.

Yingluck was ousted as prime minister days before a 2014 coup and she too faces trial in connection with accusations she mismanaged an extravagant rice-subsidy program.

The military government that has ruled since the 2014 coup has moved to suppress dissent and detained critics, many of them supporters of the Shinawatras.

Jatuporn was already on bail for another defamation case brought forward by Abhisit and a terrorism charge in 2010.

The case against him was dismissed in 2012 and 2014 when courts said his speech was considered honest and democratic criticism and therefore not defamatory.

The government has promised to hold an election next year.