Thai police today defended the arrest of the mother of a prominent anti-junta activist for allegedly insulting the monarchy, which sparked public outcry and criticism from rights groups.

Patnaree Chankij, 40, was charged yesterday for responding "yeah" in a private Facebook chat with activist Burin Intin that allegedly insults the monarch.

"She did more than just saying 'yeah' in the chat," said police colonel Olan Sukhasem at a press conference today.

"Both of them committed a crime," he said, referring to violations of a lese majeste law.

Burin, who was arrested on April 27 for posting Facebook comments that were deemed to be offensive to the monarchy, is a friend of her son Sirawit Seritiwat - one of the leaders of an anti-junta students' group.

Patnaree has been charged with lese majeste and of "computer crime," Human Rights Watch said, "for failing to criticise or take other action against Burin Intin ... who sent her Facebook messages containing alleged lese majeste comments."

Patnaree has been denied bail.

Olan did not reveal the details of the chat, but warned the public that liking, sharing or implying agreement with online content that insults the monarchy is a crime.

Thailand has a strict law that protects the king and high-ranking members of his family from defamation, insult or threat. Each offence is punishable by up to 15 years in prison.

Arrests linked to lese majeste have increased in recent years after the royalist military junta took power following a coup in May 2014.

"The junta has sunk to a new low by charging an activist's mother under the 'insulting the monarchy' law, which has been systematically abused to silence critics," said Brad Adams of Human Rights Watch.

"Prosecuting someone for her vague response to a Facebook message is just the junta's latest outrageous twist of the lese majeste law," Adams added.

The military government has been attempting to silence dissent ahead of an August referendum on its draft constitution that critics say is undemocratic and aimed at preserving its influence over the country.

- dpa