Thai lawyer and activist Anon Nampa has been arrested over a protest where he called for reform of the powerful monarchy.
Arrest warrants have been issued for five other activists.
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.
Anon, 36, was the first to openly break the taboo, earlier this month.
Insulting the monarchy in Thailand can lead to up to 15 years in jail under the kingdom’s lese-majeste law.
The use of this legislation has slowed in recent years as Thai King Vajiralongkorn has let it be known he no longer wants it so widely used.
But observers say the