Thai police on Saturday defended arresting the mother of a prominent student activist, following public outcry over a case rights groups described as a "new low" for the junta's crackdown on dissent.

Patnaree Chankij, 40, was charged Friday with violating Thailand's severe royal defamation law, which bans disparaging the monarchy with up to 15 years in prison.

Her detention followed a sweep of arrests over the past two weeks that saw at least nine people detained for criticising the junta or monarchy on Facebook.

Patnaree's lawyers said she was charged for simply writing "ja" -- the Thai equivalent of "yeah" -- in response to private Facebook messages that allegedly insulted the royal family.

"I saw the conversation and I can say that at no point did she share any opinions [of her own] about the royal family," Poonsuk Poonsukcharoen, a member of the team of human rights lawyers assisting Patnaree, told AFP.

But the police officer behind the case insisted Patnaree -- whose son is one of Thailand's most vocal junta critics -- had breached the law.

"There are also other actions involved, but we cannot reveal them," Olan Sukkasem told reporters at a press conference.

He added that "implying agreement" to unlawful web comments, through 'likes' or other means, is grounds for legal action.

The activist accused of sending the messages to Patnaree has also been detained. Both were denied bail.

Lese majeste prosecutions have sky rocketed under the royalist generals who seized power two years ago, as authorities broaden their interpretation of the crime to include even vague references to the monarchy.

A host of other harsh laws, including sedition and the broadly-interpreted computer crime act, have also been wielded against critics.

Patnaree's son Sirawith Seritiwat has been a thorn in the military's side ever since their 2014 power grab, leading small but persistent anti-junta demonstrations across Bangkok.

His mother has not participated in the protests, which are outlawed by the junta and often end in arrests.

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Human Rights Watch said Patnaree's arrest was an example of Thailand's "blatant contempt of its human rights obligations".

"The Thai 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, the group's Asia director.

Thailand's ailing 88-year-old King Bhumibol Adulyadej is the world's longest reigning monarch and revered by many Thais as a demi-god.

Observers say anxiety over succession has helped fuel the past decade of political turmoil, which the junta claims it is seeking to resolve.

The military government has become increasingly jittery ahead of an August 7 referendum on a new constitution it scripted and is determined to see pass.

Critics say the document is deeply undemocratic and aimed at enshrining the influence of the military and other traditional elites.