BANGKOK (REUTERS) - Police in Thailand said they had arrested human rights lawyer Anon Nampa for a third time this month on Tuesday (Aug 25), to charge him with sedition over his role in a political rally where calls were made for reforms to the country's powerful monarchy.

"The police brought Anon to the station to read him the charges for his protests on Aug 10, and will question him before bringing him to court to file for detention," Police Lieutenant General Amphol Buarabporn told Reuters.

The rights lawyer will be held along with another political activist, Panupong Jadnok, who was arrested on Monday during a protest against Prime Minister Prayut Chan-o-cha, who first seized power in a 2014 coup.

Protesters vowed to move ahead with a planned demonstration on Sept 19, the anniversary of a previous coup in 2006.

Police said both Anon and Panupong face charges for violating article 116, which covers sedition, and for breaching coronavirus restrictions on public gatherings.

Anon, 36, has been at the forefront of a movement that has staged protests almost daily for the past month in the South-east Asian country. He was the first to call openly for changes to King Maha Vajiralongkorn's role, breaking a longstanding taboo.

He has been arrested on similar charges twice before, and released on bail.

Police have arrested student leaders, rappers and activists for participating in demonstrations, but neither Anon or any of the other protesters have been charged under Thailand's lese majeste law, which punishes criticism of the monarchy with up to 15 years in prison.

Thousands of protesters had joined the rally on Aug 10 at a university on the outskirts of Bangkok, where protesters issued a 10-point plan for reform and also called for Prime Minister Prayut to resign.

Challenges to the monarchy were extremely rare under King Vajiralongkorn's father, King Bhumibol Adulyadej, who died in 2016 after 70 years on the throne.