The authorities in Thailand, who previously declared that marking a potentially illegal Facebook post with a "Like" could be cause for arrest, have charged a dissident's mother for failing to rebut a Facebook message that allegedly defamed the country's royal family.

Police on Saturday sought to defend the arrest of Patnaree Chankij for allegedly violating the law on lese majeste -- defaming the monarchy -- and the Computer Crime Act. They told reporters they had other evidence against her, but would not give details.

The group Thai Lawyers for Human Rights, which is defending both Patnaree and her son, said Friday that the police allege that the 40-year-old widow and mother of three failed to condemn a message she was sent by a man recently charged with lese majeste, which is punishable by three to 15 years in prison.

Patnaree's son, Sirawith Seritiwat, is a leader of a small group of activists who have repeatedly staged demonstrations against Thailand's repressive military junta, which took power in May 2014 after toppling an elected government. He has been detained and arrested numerous times.

His mother's arrest was widely seen as a way of putting pressure on him. The complaint against Patnaree was filed with police by the military.

"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, Asia director at 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."

The police charge sheet, posted by her lawyers, alleged that in an online dialogue that allegedly touched on the monarchy, Patnaree -- under the name "Nuengnuch Charnkij" -- used the Thai word "ja," which commonly would be understood as an acknowledgment rather than an endorsement.

The police alleged that her use of the word showed that she agreed with the post, which police declared "can be harmful to the Kingdom's security and stability ..."

"If Nuengnuch disagreed with his post, Nuengnuch should condemn or stop him from posting such a message. Instead, she replied 'Ja' to him, which means that she agrees with him."

Thai media reported that Patnaree's home in a Bangkok suburb was raided early Saturday afternoon and computers were taken away.

Her arrest is the latest in a series that began last month for people who posted online material critical of the country's ruling junta and its policies. Several arrests have also been based on texts sent privately over Facebook Messenger, causing alarm that users' privacy is being violated.

Users are also disturbed that Facebook has blocked a site that allegedly mocked the monarchy, the first time it appears to have done so in Thailand. The page redirects to an announcement that it is blocked to be in compliance with Thai law. In response to the Facebook issues, several activists have started a campaign to stop using the huge social network in favor of a competitor.