The Thai government has announced an emergency decree to stem largely peaceful protests in Bangkok, including a ban on large gatherings.
In a televised announcement read out by police it said urgent measures were needed to “maintain peace and order”.
On Thursday morning police arrested several activists, including three key protest leaders.
The student-led democracy movement has called for the prime minister to resign and curbs on the king’s powers.
Police said they arrested about 20 people, but did not confirm their names.
The BBC has learned that those arrested include key protest leaders – the human rights lawyer Anon Nampa, student activist Parit Chiwarak, widely known by his nickname “Penguin”, and Panusaya Sithijirawattanakul.
In a widely-watched livestream video police officers were seen reading out charges to Ms Panusaya in a hotel room. Another video showed police putting her into a car as she and her supporters chanted slogans.
Mr Anon, 36, was the first to openly break the taboo on discussing Thailand’s monarchy by calling for reforms in August. Ms Panusaya became one of the most prominent faces of the protests since she delivered a 10-point manifesto urging royal reform later that month.
Both men have been arrested previously over the student-led protest movement that has swept Bangkok since it gained momentum in July. Ms Panusaya, 21, had not been arrested until now.