BANGKOK (AFP) - The Thai police have arrested a leading activist's mother for failing to condemn Facebook messages that criticised the monarchy, her legal team said on Saturday (May 7), in a move a rights group described as an "outrageous twist" of the kingdom's royal insult law.

Prosecutions under Thailand's royal defamation law - known as lese majeste - have surged since the generals seized power two years ago. The boundaries for what counts as an insult have also expanded dramatically, with the authorities making arrests over even vague references to the royal family.

But human rights lawyers said the police took a step further Friday night by charging Ms Patnaree Chankij, 40, for simply receiving messages that criticised the monarchy in a private Facebook chat.

"She just received the messages. She didn't say anything in her own words about the royal family," Mr Poonsuk Poonsukcharoen, one of the human rights attorneys representing Ms Patnaree, told AFP.

Her 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," said Mr Poonsukcharoen, a member of the team of human rights lawyers assisting Patnaree.

But the police officer behind the case insisted Ms Patnaree had breached the law. "There are also other actions involved, but we cannot reveal them," Mr 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.

Police denied a request for bail citing the seriousness of the crime, which carries up to 15 years in prison for each offence.

She is scheduled to appear before a military tribunal on May 8, the court told AFP.

Ms Patnaree is the mother of prominent student activist Sirawith Seritiwat, who has been at the fore of small but persistent anti-junta demonstrations.

She herself has not participated in the protests, which are banned by the junta and often end in arrests.

Human Rights Watch said the charge against her was a "new low".

"Prosecuting someone for her vague response to a Facebook message is just the junta's latest outrageous twist of the lese majeste law," said Mr Brad Adams, the group's Asia director.

At least nine people have been detained in jail in the past two weeks for online criticism of the junta or monarchy - including an activist who allegedly sent the messages to Ms Patnaree.

The arrest comes amid a heightened crackdown on dissent by the junta ahead of an August 7 referendum on a new Constitution it scripted and is determined to see pass.

The military government says it is seeking to end the political turmoil that has dogged the kingdom for the past decade.

But critics say the generals are more concerned with enshrining their influence and weakening the political bloc led by ex-premier Thaksin Shinawatra.

Thaksin was ousted in a 2006 coup and is hated by Bangkok's military-backed elite.

But his parties have continued to dominate elections with broad support from people in the country's rural north.