Thailand has imposed a state of emergency in its capital, Bangkok, and surrounding areas.
The action by Prime Minister Abhisit Vejjajiva comes a day after anti-government protesters swarmed into a hotel hosting an Asian summit, forcing its cancellation.
The protesters, who are seeking Abhisit's resignation, also blocked a major Bangkok motorway for several days last week, disrupting traffic.
The decree bans gatherings of more than five people and forbids reporting that is considered threatening to public order.
Bands of red-shirted anti-government protesters roamed areas of Bangkok as the emergency decree was announced, with some smashing a car they believed was carrying Vejjajiva, and others attacking motorists who hurled insults at them.
The emergency decree also allows the government to call up military troops to quell unrest.
"The government decided to impose the state of emergency because we want to return the country to normalcy," Abhisit said on national television.
"The government will try every way to prevent further damage. I ask the people to support the government in order to restore order in the country."
Earlier, a protest leader who spearheaded Saturday's demonstrations, Arisman Pongruengrong, was taken into custody and flown by helicopter to a military camp for questioning, said police spokesman Maj Gen Suport Pansua.
Abhisit also vowed swift legal action against other protesters who stormed the venue of an East Asian Summit in the beach resort of Pattaya on Saturday, forcing the summit's cancellation. Thai authorities had to evacuate the Asian leaders by helicopter.
