The mother of a prominent student activist was indicted by a Thai military court on royal defamation charges Monday over a one-word Facebook message in what rights groups have described as a new low for the junta's crackdown on dissent.

Patnaree Chankij, 40, was initially arrested in May by Thai police for violating Thailand's severe royal defamation law, which bans disparaging the monarchy with up to 15 years in prison on each count.

Her lawyers say the charge stems from writing "ja" -- the Thai equivalent of "yeah" -- in response to a private Facebook message from someone else that allegedly insulted the royal family.

The case has been seized upon by activists as a terrifying example of how the military authorities have broadened their interpretation of the crime to include even vague references to the monarchy.

Police eventually dropped the charges after an outcry but they forwarded the case to prosecutors from the military who have pressed ahead.

"A military court today accepted the case filed by the military prosecutor (on 112)," lawyer Anon Nampa, told AFP, adding his client was granted bail ahead of an upcoming plea hearing.

Use of both Thailand's lese majeste law and military courts has skyrocketed since former army chief turned Prime Minister Prayut Chan-O-Cha seized power in May 2014 sparking international condemnation, including from the United Nations.

Record breaking 25 and 30 year sentences have since been handed down for people over Facebook posts while a man even found himself arrested for making sarcastic comments about the king's late dog.

Many convictions occur behind closed doors while media must also heavily self-censor when reporting cases to avoid falling foul of the law.

Reporters were not allowed into the military court on Monday and the court did not answer telephone calls.

Police had earlier defended Patnaree's arrest telling reporters that "implying agreement" to unlawful web comments, through 'likes' or other means, is grounds for legal action.

The latest indictment comes ahead of a crunch referendum vote this weekend on the junta's new constitution, the first time Thais will head to the polls since the coup.

The generals say their charter -- the kingdom's twentieth -- will curb political corruption and bring much-needed stability after a decade of political turmoil.

But critics deride it as a naked attempt to perpetuate the military's grip on power.

Security is being ramped up ahead of Sunday's vote.

Earlier Monday police announced that they had launched a pre-referendum crime crackdown in the northeast, an impoverished but populous region where many of the junta's fiercest critics reside.

In a statement released Monday Thai police said the crime blitz had netted some 240 people who were caught with weapons, 1,908 who were detained on drugs charges and a further 1,488 who were arrested for gambling.