Thai King Bhumibol Adulyadej
Thailand's King Bhumibol Adulyadej is seen attending a ceremony in Bangkok, Thailand, Dec. 14, 2015, in this still image taken from Thai TV Pool video. Reuters/Thai TV Pool

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.

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 214 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.

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.

Police Colonel Olarn Sukkasem confirmed the arrest warrant was issued on lese-majeste charges, but declined to give details.

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.

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 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.