BANGKOK (Reuters) - The mother of a leading activist against the military junta in Thailand was charged on Monday with insulting the country's monarchy in a one-word Facebook post.

Patnaree Chankij was brought to a military court in Bangkok after the country's attorney general decided to press charges despite police saying earlier that they would not pursue a case against the 40-year-old woman.

According to her lawyer, Patnaree was charged with violating Thailand's royal insult laws for writing the word "ja" - which means "yeah" in Thai - in response to a private Facebook message critical of the royals.

She was released on bail.

"The court accepted the case from the attorney general and freed Patnaree on bail," said Anon Numpa, her lawyer.

Under Article 112 of the criminal code, anyone who "defames, insults or threatens the king, queen, heir-apparent or regent" faces up to 15 years in prison.

The case has drawn international criticism since May, when police first issued an arrest warrant for Patnaree and charged her with defaming the monarchy. The police subsequently said they would drop the charges.

The United States and several rights organizations, including the New York-based Human Rights Watch, condemned Patnaree's arrest and the charges brought against her. The State Department in May said it created a "climate of intimidation".

The junta has clamped down on dissent ahead of a referendum next week on whether to accept a military-backed constitution that critics say is designed to enshrine military power.

Patnaree's son, Sirawith Seritwat, is a student activist with the New Democracy Movement and Resistant Citizen, groups that the authorities have regularly targeted because of their activities, including handing out leaflets urging people to reject the draft constitution.

During its two-year rule, the military government has taken a hardline stance against perceived royal insults and has handed down record sentences.

Prime Minister Prayuth Chan-ocha, who also heads of the junta, has said he would show zero-tolerance to insults of the monarchy.

For more than a decade, Thailand has been bitterly divided between rival camps, one led by former populist premier Thaksin Shinawatra, who was toppled in a 2006 coup, the other dominated by the royalist and military establishment who accuse Thaksin of corruption and nepotism, charges he denies.

National anxiety over the frail health of 88-year-old King Bhumibol Adulyadej has compounded the political tensions. Thais mostly see the king as a unifying force and celebrated the 70th year of his reign in June.

(Reporting by Amy Sawitta Lefevre and Aukkarapon Niyomyat; Editing by Simon Cameron-Moore)