An activist and lawyer made famous for his open calls for reform of Thailand's powerful monarchy was on Tuesday sentenced to four years in prison for royal insults, a judge said, in one of the country's most high profile lese-majeste cases.

Human rights lawyer Arnon Nampa, 39, is widely known for his taboo-breaking speech during pro-democracy protests in 2020 during which he called for public debate on the role of Thailand's king. Arnon denies wrongdoing.

Thailand's lese-majeste law shields the palace from criticism and carries a maximum jail sentence of 15 years for each perceived insult of the monarchy, a punishment widely condemned by international human rights groups as extreme.

Arnon was a leader of a youth-led democracy movement that held protests in Bangkok in 2020 that drew hundreds of thousands of people demanding the removal of royalist former prime minister Prayuth Chan-ocha, who seized power in a coup.

Arnon was found guilty over remarks about the monarchy at a speech during a 2020 rally, in the first of 14 cases against him for violating article 112 of the criminal code, as the royal insults law is known.

"Arnon will be sent to prison while he waits for a bail decision, which could take two to three days," lawyer Krisadang Nutcharus told Reuters, adding his team would lodge an appeal and if necessary, take the case to the Supreme Court.

Arnon has been on bail since early last year after several periods of detention.

Hundreds of people have been charged under article 112, which is among the world's strictest royal insults laws, with some violators given sentences of decades, including a 64-year-old woman jailed for 43 years. The palace typically does not comment on the law.

UNTHINKABLE MOVES The verdict against Arnon will be a setback for groups seeking amendments to article 112, moves that were unthinkable only a few years ago in a country where the constitution states the king is "enthroned in a position of revered worship".

Calls for the law to be changed were central to a bold, anti-establishment platform that saw Thailand's progressive Move Forward Party win an election in May, only to be blocked from forming a government by lawmakers backed or appointed by the ultra-royalist military.

According to legal aid group Thai Lawyers for Human Rights at least 257 people have been charged under article 112 in the past three years.

Most of those cases are related to the youth-led democracy movement, which has since lost momentum having once posed one of the biggest challenges to Thailand's royalist, conservative establishment.

In remarks as he arrived at court, Arnon acknowledged he would likely lose his freedom and said he had no regrets for what was "a worthwhile personal sacrifice for the greater good".

"The youth protest has created a phenomenon that has changed Thailand to the point of no return," he said.

"I believe that the people are becoming more confident in their freedom and equality and are ready to transform the country to be more progressive."