BANGKOK >> The mother of a pro-democracy activist faces up to 15 years in prison after acknowledging that she had received a private message on Facebook that the police say insulted Thailand’s monarchy.

The activist’s mother, Patnaree Chankij, 40, who works as a maid, will be tried by a military court under Thailand’s lèse-majesté law, which makes it a crime to insult the long-reigning King Bhumibol Adulyadej, the queen or the crown prince.

On Saturday, human rights activists called Patnaree’s arrest a day earlier a “new low” for Thailand, which has increased prosecutions under the lese-majeste law since the military took power in 2014.

Poonsuk Poonsukcharoen, a lawyer advising Patnaree, said Patnaree had sent only a one-word reply, “Ja,” acknowledging receipt of the Facebook message, similar to saying “Yeah,” but had not expressed agreement with it or commented on its content. The message was not made public, so as not to repeat the alleged insult, as is typical in such cases. The sender of the message, Burin Intin, 28, was arrested last month.

Col. Olarn Sukkasem, the chief of the police’s Technology Crime Suppression Division, told reporters that the authorities had enough evidence to show that the pair had “carried out things together.” He did not provide specifics, saying the investigation was continuing.

Human Rights Watch said Saturday that the military junta in Thailand had “arbitrarily and aggressively” used the lèse-majesté laws to prosecute people for any speech deemed critical of the monarchy.

Since the coup two years ago, the authorities have brought 57 cases under the law, 44 of them involving online commentary, the rights group said.

Among those who have been investigated under the law are a factory worker accused of insulting the king’s dog; a scholar accused of insulting a king who died centuries ago; and U.S. Ambassador Glyn T. Davies, who, despite his diplomatic immunity, was investigated after he criticized long prison sentences handed out under the law. One man is serving 30 years in prison for posts he distributed on Facebook.

“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 Asia director 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.”

At a news conference Saturday, Olarn advised the public to exercise caution when communicating online.

“If you share this offending information, you must take responsibility for your act, as well,” he said. Simply clicking “Like” on Facebook could be considered an offense, he said.

He also had a warning for journalists reporting on Patnaree’s arrest. “Right now, it is under investigation,” he said. “Therefore, I would like you to be careful in publishing information. Publishing false information can be considered an offense.”

Patnaree was denied bail because the severity of her potential sentence made her a flight risk, Poonsuk said. Patnaree’s son, Sirawith Seritiwat, a leading student activist, is out on bail on charges that he took part in a banned protest.

At a protest outside a Bangkok police station Saturday evening, Sirawith told his mother’s supporters and the news media that the police had gone after her to get to him.

“She’s never expressed anything politically,” he said. “She’s never argued or debated with anyone.”

He added: “The junta is using my mother as a hostage. And I can tell you that this is not a manly act. And this is not an act that a government should be doing to its people.”

Poonsuk, who is not officially representing Patnaree but accompanied her when she turned herself in, said acknowledging a message with a single word was not evidence that she had insulted the king or conspired with Burin to commit a crime.

“I haven’t seen any legal interpretation like this,” she said. “Just saying ‘yeah’ doesn’t mean that she should be considered an accomplice.”

© 2016 The New York Times Company