Prawet Prapanukul, 57, provided legal assistance to members of a political opposition group, the United Front for Democracy Against Dictatorship, and served as legal counsel in a high-profile royal insult case.
He was detained in a raid on his home by police and troops last weekend.
Prawet who appeared in a Bangkok court was charged with 10 counts of royal insult and three counts of breaking Section 116 of Thailand’s criminal code, the equivalent of sedition.
“Prawet faces 10 counts of breaking Article 112, so that works out to up to 150 years in prison if he is found guilty.
“He is also facing three separate charges of breaking Article 116 which carries a maximum sentence of seven years in prison for each offence,” Anon Nampha of Thai Lawyers for Human Rights said.
It was not immediately clear what Prawet had written or said that led to his arrest and charges.
A spokesman for the military government said he was unable to comment on the case.
Thailand’s military seized power from an elected civilian government in a May 2014 coup.
Since then, the junta has detained hundreds of politicians, activists, journalists and others accused of being involved in anti-junta protests and activities.
Some have been held incommunicado in unofficial places of detention, such as military camps.
It has also taken a hardline stance against suspected transgressors of Article 112 of the criminal code which makes anyone found guilty of insulting the monarchy liable to imprisonment for up to 15 years.
The lawyers’ group said Prawet was charged along with five others, who also faced charges under the royal insult law.
Prawet is in pre-trial detention at a Bangkok remand prison after being held incommunicado at the 11th Army Circle base in Bangkok, a facility the military uses as a temporary prison.
The group Human Rights Watch urged the military to observe due process.
“Even though lese majeste is a serious criminal offense in Thailand, authorities have no justification to breach due process and fair trial standards.
“The military regime has aggressively used lese majeste charges to clamp down on any speech and opinion they find objectionable,” Sunai Phasuk, Thailand researcher at Human Rights Watch said.
Lese majeste laws have an impact on what any news organisation, including Reuters, can report about issues relating to Thailand’s monarchy.
King Maha Vajiralongkorn, 64, ascended the throne in December following the death of his father, King Bhumibol Adulyadej, in October 2016.
Thailand is observing a year of mourning for King Bhumibol who was seen as semi-divine by many people.
His cremation will be held in October.
Political activity has largely ceased to mark the period of mourning.
The government has said that an election to restore democracy will be held in 2018, although a date has not been fix.