Police fired teargas on protesters who hurled stones and petrol bombs in demonstrations that paralysed parts of Bangkok and followed a night of gun and knife battles in which two people were killed and at least 54 wounded.
A group of protesters forced Prime Minister Yingluck Shinawatra to evacuate to an undisclosed location from a building where she had planned to give media interviews, while hundreds seized control of state broadcaster Thai PBS, waving flags and tooting whistles.
Declaring Sunday "V-Day" in a week-long bid to topple Yingluck and end her family's more than decade-long influence over Thai politics, protest leaders urged supporters to seize 10 government offices, six television stations, police headquarters and the prime minister's offices in what they are calling a "people's coup".
Police said the protesters had gathered in at least eight locations. In at least three of them, police used teargas and water canons.
National police spokesman Piya Utayo said troops were being sent to a government complex occupied by protesters since Thursday and the Finance Ministry, occupied since Monday. "We have sent forces to these places to take back government property," he said on national television.
It is the latest dramatic turn in a conflict pitting Bangkok's urban middle class and royalist elite against the mostly rural poor supporters of Yingluck and her billionaire brother, Thaksin Shinawatra, a former prime minister ousted in a 2006 military coup.
Reuters journalists waiting to interview Yingluck inside the police Narcotics Suppression Bureau were told by Natthriya Thaweevong, an aide for the prime minister, that she had left after protesters made it inside the outer part of the compound, the Police Sports Club, where the bureau is located.
In the early afternoon, protesters massed in front of a police barricade outside Wat Benjamabhopit, also known as the Marble Temple. Police fired teargas as some protesters tried to heave aside the heavy concrete barriers.
The deep detonation of stun grenades, followed by the jeers of protesters, echoed across the historic quarter.
"I just want the people named Shinawatra to get on a plane and go somewhere - and please, don't come back to our country again," said Chatuporn Tirawongkusol, 33, whose family runs a Bangkok restaurant.
Outside the Metropolitan Police Bureau, about 3,000 protesters rallied, accusing riot-clad police of being manipulated by Thaksin, a former policeman who rose to become a telecommunications magnate before entering politics and winning back-to-back elections in 2001 and 2005.
Chamai Maruchet Bridge, north of Government House, the prime minister's offices, was a scene of nearly non-sop skirmishes, as police repeatedly fired teargas into the stone-throwing crowd, Reuters witnesses said. Protesters gathered near barricades spray-painted with the words "Failed State".
A Reuters photographer saw protesters hurl at least a dozen petrol bombs into police positions from a college campus across a canal from Government House.
In one of the most dramatic events, state broadcaster Thai PBS was taken over by protesters, according to PBS and police. More than 250 mostly black-shirted protesters gathered in the parking lot, as others streamed in.
The executive producer at Thai PBS, Surachai Pannoi, told Reuters the management of the station would share its broadcast line with Blue Sky, a broadcaster controlled by the opposition Democrat Party, starting this afternoon.
Yingluck, who won a 2011 election by a landslide to become Thailand's first female prime minister, has called for talks with the protesters, saying the economy was at risk after demonstrators occupied the Finance Ministry on Monday.
Protest leader Suthep Thaugsuban, a deputy prime minister under the previous Democrat-led government that Yingluck's party routed 2011, has ignored her and told demonstrators that laws must be broken to achieve their goals.
The Democrats, Thailand's oldest political party, have not won an election in more than two decades and have lost every national vote for the past 13 years to Thaksin or his allies.
Suthep has called for a "people's council", which would select "good people" to lead, effectively suspending Thailand's democratic system. Yingluck has rejected that step as unconstitutional and has ruled out a snap election.
Thailand faces its worst political crisis since April-May 2010, a period of unrest that ended with a military crackdown.
In all, 91 people were killed then, mostly Thaksin's supporters trying to oust the then-Democrat government. Suthep faces murder charges for his alleged role in the ordering crackdown.
Police tightened security after clashes on Saturday between supporters and opponents of Yingluck near a sports stadium where about 70,000 red-shirted government supporters had gathered.
Five big shopping malls closed their doors in Bangkok, underscoring the economic impact of the protests. One "red shirt" government supporter was shot and killed outside the stadium early on Sunday, after a 21-year-old student was fatally shot several hours earlier.
A red-shirt leader, Jatuporn Promphan, said four red shirts had been killed but Reuters only confirmed one, 43-year-old Viroj Kemnak. Fifty-four people were wounded, according to the government's Erawan emergency centre.
Thousands of government supporters began to disperse, returning on buses to their homes in the north after their rally was called off in a bid to defuse tensions.
Seventeen battalions of 150 soldiers each, along with 180 military police, all unarmed, were called in to boost security ahead of the demonstrators' Sunday deadline for ousting the government.
Thaksin, who won over poor rural and urban voters with populist policies, was convicted of graft in 2008. He dismisses the charges as politically motivated and remains in close touch with the government from his self-imposed exile, sometimes holding meetings with Yingluck's cabinet by webcam.