Pathumwan District Court on Monday sentenced Pita Limjaroenrat, Thanathorn Juangroongruangkit and six others each to four months in prison, suspended for two years, over a flash mob protest in Pathumwan district in 2019.
Mr Pita is chief adviser of the opposition-core Move Forward Party, Mr Thanathorn is the former leader of the disbanded Future Forward Party (FFP) and head of the Progressive Movement.
The court also passed sentence on Piyabutr Saengkanokkul, secretary-general of the movement, Pannika Wanich, the spokeswoman, Pairattachote Chantarakajon, former election candidate of FFP, Nattha Mahatthana, Thanawat Wongchai and Parit Chiwarak.
They were all found guilty of organising a flash mob on the skywalk near the train station at Pathumwan intersection and within 150 metres of the Sa Pathum Palace, affecting train services and preventing people from using a public place.
The court sentenced the defendants each to four months in prison, suspended for two years because they had not previously been sentenced to jail and the protest had been for the purpose of political expression and was not a severe crime.
The court also fined them 20,200 baht for staging the rally without prior notice and using loudspeakers without a permit.
Mr Pita said he and his co-defendants would appeal against the ruling, on the issue of the rally's distance from the palace.
The flash mob gathered on Dec 14, 2019. It was the biggest protest since the 2014 coup and followed the Election Commission's decision to seek the disbandment of the Future Forward Party for illegally accepting a large loan from Mr Thanathorn.
The court subsequently dissolved the party and banned Mr Thanathorn and other members of the executive from holding political office.