Thailand's Prime Minister Samak Sundaravej has declared a state of emergency in the capital Bangkok, after clashes left at least one person dead, reported BBC.
Dozens more were hurt in the fighting early on Tuesday.
The clashes began after supporters of Mr Samak pushed through police lines near Government House, occupied by the People's Alliance for Democracy.
PAD supporters say the government is a front for the exiled former Prime Minister, Thaksin Shinawatra.
"Because last night there was unrest affecting the peaceful law and order in the country and obstructing the democratic process, the government has declared a state of emergency, which will affect people's individual freedoms," an announcement on state radio said.
Mr Samak put the army commander, General Anupong Paojinda, in charge of enforcing the state of emergency in the capital. He now has special powers to:
detain and remove people from any location
deploy troops to help police maintain order
ban gatherings of more than five people
impose restrictions on media reports that could "undermine public security".
The PAD leader said protesters occupying the government's building had refused to move, despite the state of emergency.
"There are not enough jails to put us all into," Chamlong Srimuang told thousands of PAD demonstrators.
An emergency debate in parliament earlier failed to end the stalemate between Mr Samak's fragile coalition and his opponents.
Overnight, thousands of PAD supporters took to the streets of Bangkok and forced their way into government buildings.
Mr Samak was initially praised for his restraint in dealing with the protests, but he suddenly found his position weakened on Friday when the police pulled back and the triumphant protest leaders were left in control of the government complex.
The demonstrations in the capital remained relatively peaceful over the weekend, but early on Tuesday violent clashes broke out when hundreds of government supporters marched towards Government House, armed with sticks and clubs.
TV pictures showed a man lying motionless on the ground with a pool of blood around his head as well as dozens of injured afterwards.
Dr Petchpong Kamjornkijjakarn of the Erawan Medical Centre said one person had died from severe head injuries and that four others were in a serious condition, two of whom had gunshot wounds.
At least 31 others are being treated for injuries across the city.
On Monday, public sector unions said they would begin a nationwide strike at 43 state enterprises on Wednesday unless the prime minister stepped down.
The State Enterprise Workers Relations Confederation also threatened to cut off water, electricity and phone service at government offices and disrupt flights.
The BBC's Jonathan Head in Bangkok says there appear to be very few ways out of this crisis.
The government retains strong rural support and would probably be re-elected if fresh polls were called - and the PAD does not support such elections, our correspondent says.
Meanwhile, the revered King Bhumibol has also refused to intervene.
The PAD - a loose grouping of royalists, businessmen and the urban middle class - was formed in the months before Mr Thaksin was ousted by a military coup in 2006. He has since fled the country to avoid facing trial over corruption charges.
It has a passionate following in various parts of the country, especially Bangkok, and some powerful backers among the elite.
But it has little support in most of rural Thailand, which voted strongly for Prime Minister Samak, and Mr Thaksin before him.