BANGKOK (REUTERS, AFP) - A Thai prosecutor on Monday (March 8) charged 18 activists over their roles in anti-government rallies last year by a protest movement that brought unprecedented challenges to the royal palace and military-dominated establishment.

The youth-led movement sprang up last year calling for the resignation of Prime Minister Prayut Chan-o-cha, a former junta leader, and reform of the powerful Thai monarchy, breaking a longstanding taboo under the country's lese majeste law.

Those indicted included three prominent leaders charged with sedition and lese majeste during rallies in September last year, where tens of thousands escalated calls for monarchy reforms.

The other 15 protesters face trial for sedition and breaching a ban on public assembly.

"There is sufficient evidence that the accused have committed wrongdoing," said the Attorney-General office's spokesman Chanchai Chalanonniwat.

The three prominent activists - Panusaya "Rung" Sithijirawattanakul, Panupong "Mike" Jadnok and Jatupat "Pai" Boonpattararaksa - were on Monday denied bail and remanded in custody.

Thailand's lese majeste law prohibits criticism or insults against the king, and each offence is punishable by up to 15 years in prison.

Panupong said his group was unfazed by the prospect of jail.

"I am not too worried," Panupong said. "The activities we have done are only the beginning, and it will go ahead even without us."

Thailand's youth movement has posed the biggest challenge so far to former army chief Prayut, whom the protesters said engineered the rules of the 2019 election to keep himself in power.

Protesters also said the Constitution gives the King too much power, and demanded that some of it be curbed.

Four other activist leaders are in jail awaiting trial over the same protests, having been denied bail five times.

At least 63 people have been charged under lese majeste laws since November, according to the Thai Lawyers for Human Rights group.