Thai Lawyer and activist Anon Namfa has been arrested over a protest where he called for reform of the powerful monarchy.
Thailand’s monarchy has long been shielded from criticism under strict lese-majeste and other laws.
But in recent weeks anti-government rallies have started calling for change to an institution Thais are taught to revere from a young age.
Mr Anon, 36, was the first to openly break the taboo earlier this month.
Four other Thai activists have also been arrested over a student led protest in July that sparked this latest wave of demonstrations.
Protesters are demanding dissolution of the government headed by Prime Minister Prayuth Chan-ocha, the former Army Chief who seized power in a 2014 military coup and returned as the country’s civilian premier after a controversial election last year.
The other pro-democracy activists arrested on Wednesday and Thursday includes a rapper who performed at the July protest at Bangkok’s Democracy monument.
Thai Police are cracking down on the student led monument that has given rise to the country’s biggest rallies since the coup six years ago.
Protesters seek the resignation of Military backed government, an end to the harassment of its critics, a new constitution and fresh elections.