BANGKOK (Reuters) - The mother of one of Thailand’s highest-profile anti-junta activists has been charged with defaming the monarchy, police said on Friday, in what a rights group said was an escalation in government attempts to stifle dissent.

Thailand's Prime Minister Prayuth Chan-ocha looks on before a weekly cabinet meeting at Government House in Bangkok, Thailand, April 26, 2016. REUTERS/Athit Perawongmetha

Political tension is building ahead of an Aug. 7 referendum on a military-backed constitution, which is the first vote under the junta and a test of Prime Minister Prayuth Chan-ocha’s popularity. His government has threatened to jail anybody campaigning against the charter.

Patnaree Chankij turned herself in on Friday after police issued an arrest warrant. In comments to TV reporters, Patnaree denied she had committed lese-majeste.

The junta, which took power in a May 2014 coup, has been cracking down on critics of the monarchy using Thailand’s strict lese-majeste law - a French term for the crime of offending the dignity of a sovereign.

Anon Numpa, one of the two lawyers representing Patnaree, wrote on his Facebook profile that his client has been charged by the authorities for not responding or discouraging another Facebook user from writing an offending message on a private Facebook chat.

“Someone sent you messages that may violate article 112, and you saw them, yet did not stop or prevent them, is equal to you participating in the offence,” said Anon in a Facebook post.

“Are we really going to walk this path?” he said.

He referred to the article 112 of Thailand’s criminal code which says anyone who “defames, insults or threatens the king, the queen, the heir-apparent or the regent” will be punished with up to 15 years in prison.

Last year, there was international condemnation when two people received jail sentences of 25 years and 30 years on lese-majeste charges for Facebook posts about the king.

Patnaree’s son is Sirawith Seritiwat, one of the leaders of a student anti-coup group and a consistent government critic. He says he has been detained and arrested at least 10 times since the army seized power.

Thai Lawyers for Human Rights, an organization that represents Patnaree, said on Twitter that she would be brought to a military court on Sunday.

The charges against his mother were an example of how the military is using lese-majeste laws as a political tool, he told Reuters.

“This is a marked escalation of the junta witch-hunt against dissidents,” said Brad Adams, Asia director at Human Rights Watch. “Now they are targeting a dissident’s mother as they try to gag his outspoken criticism. This is a new low even for a junta which has made abuses of human rights a daily occurrence.”

The junta has muted opposition with a ban on political gatherings of more than five people, and stifled freedom of expression by detaining academics, activists and other critics for what are referred to as attitude adjustment sessions.

That has given a semblance of calm to a nation bitterly divided for over a decade between the royalist-military establishment and the supporters of former Prime Minister Thaksin Shinawatra, a telecommunications billionaire ousted in a 2006 coup.

“Two years of junta rule seems peaceful, but actually the problems haven’t decreased,” said academic Anusorn Unno of the Thai Academic Network for Civil Rights.

Junta spokesman Winthai Suvaree said the academics did not represent the views of majority of Thais.