Weerayuth Kongkanathan, 49, was given a one-month suspended jail term after being found guilty of violating martial law by protesting against the coup launched by Thailand's army on May 22.
"He was found guilty of violating the order prohibiting political gatherings... The first one to be convicted under the NCPO (junta body) order," Pawinee Chumsri, one of the lawyers representing Weerayuth, told AFP.
Political assemblies of more than five people were banned under martial law declared by army chief Prayut Chan-o-Cha two days before he ousted the kingdom's caretaker government.
The city's Pathumwan Municipal Court sentenced him to two months in jail and imposed a fine of USD 185 -- both of which were reduced by half when Weerayuth pleaded guilty.
The anti-coup activist was relieved to escape jail but vowed to continue resisting military rule, despite facing prison if he participates in further demonstrations.
"I will not stop, I have to avoid being arrested again," he said. "I am a democracy lover, why do I have to be afraid?"
Rights groups today called for Weerayuth's conviction and sentence to be expunged, warning that it set a dangerous precedent.
"The verdict sends an alarming message that peaceful protest is not tolerated in Thailand," Richard Bennett, Amnesty International's Asia Pacific Director, said in a statement.
"This is likely to be the first in a series of planned trials of people who have voiced dissent against the army's rule. We are calling for it to be the last."
