It is the first time in more than two years that anyone has been charged with lese majeste, Article 112 in the Thai criminal code, which can mean a jail sentence of up to 15 years.

"112 is an unjust law. I don't give it any value," rights lawyer and protest leader Arnon Nampa told reporters as he arrived at the police station on Monday. "I am ready to fight in the justice system."

Arnon was joined by Panupong "Mike Rayong" Jadnok, Panusaya "Rung" Sithijirawattanakul and Parit "Penguin" Chiwarak.

They did not say whether they would plead guilty. A total of seven protest leaders face royal insult charges. They and dozens of other protesters face other charges relating to demonstrations since July.

The protests have become the biggest challenge to the monarchy in decades as they have broken taboos by openly criticising a monarchy that must be revered according to the constitution.

The Royal Palace has not commented since protests began. When asked about the protesters recently, the king said they were loved "all the same".

Protesters have called for the king's powers to be curbed so that he is clearly accountable under the constitution. They also seek to reverse changes that gave him control of the royal fortune and some army units.

Critics of the monarchy say it has enabled decades of domination by the military, which has carried out 13 successful coups since the end of absolute monarchy in 1932.

Prime Minister Prayuth Chan-ocha, a former army chief who overthrew an elected government in 2014, had said in July that the lese majeste charges were not currently being used at the request of the king.

"Using 112 against all of us shows the world and Thai society that the monarchy is now the political opposition," Parit said.

Protesters seek the removal of Prayuth, accusing him of engineering last year's election to keep hold of power. He says the vote was fair. Protesters also want to replace the constitution, which was drawn up by Prayuth's former junta and then modified by the king.

Australian Associated Press