The protest is one of a series of mass demonstrations against the country’s military-backed establishment.
The 22 demonstrators deny charges of committing sedition and a litany of other offences, which includes lese majeste, a crime punishable by up to 15-year imprisonment for each count.
“They can lock me up but cannot lock up the truth,” protest leader Parit Chiwarak shouted as he arrived in a prison truck, defiantly flashing the three-finger “Hunger Games” salute synonymous with the youth movement.
“The truth is always the truth whether in prison, under torture, or awaiting execution, the truth is the truth,” Mr. Parit added.
The accused who is a 22-year old is also among seven defendants held in pre-trial detention and accused of insulting King Maha Vajiralongkorn, as well as sedition.
Thailand’s youth movement has posed the biggest challenge so far to the Prime Minister and former coup leader Prayuth Chan-ocha.
Mr. Chan-ocha was said to have engineered a process that would preserve the political status quo and keep him in power after the 2019 election, however, Mr. Parit has rejected that.
Protesters also broke a traditional taboo by demanding reform of the powerful monarchy, saying the constitution drafted by the military after the 2014 coup gives the king too much power.
The length of the trial will be determined after the defence and prosecution discuss how many witnesses both sides will call upon for the case, which stems from a September rally.
