Bangkok and about a third of the rest of Thailand have spent a first night under curfew after street protest leaders surrendered.
At least 27 buildings were set ablaze after the red-shirt leaders' surrender and pockets of resistance remained despite pleas from leaders to go home.
Some 40 people have died since troops ringed the protesters last week, with at least six more deaths on Wednesday.
Fires were reported at the stock exchange, banks and a shopping mall.
A huge blaze engulfed Central World, one of South-East Asia's biggest shopping centres.
Appearing on TV, Prime Minister Abhisit Vejjajiva said he was "confident and determined to end the problems and return the country to peace and order once again".
Thaksin Shinawatra, the former prime minister living in self-imposed exile whom many red-shirts support, said the crackdown could spawn mass discontent and lead to guerrilla warfare.
"There is a theory saying a military crackdown can spread resentment and these resentful people will become guerrillas," he told Reuters news agency by telephone.
The curfew, the first imposed in the city in 15 years, ran from 2000 to 0600 (1300 to 2300 GMT), and the government ordered television channels to broadcast only officially sanctioned programmes.
The protest may be over but the bloodshed will not be forgotten and the bitterness and anger linger on, reports the BBC's Rachel Harvey from Bangkok.
Thailand's deep divisions have been brutally exposed, our correspondent adds.
There are reports of tension in the north and one group operating in the capital has declared itself independent of the main protest movement and says it will continue fighting.
In the north-east of the country, a town hall in Udon Thani was set on fire and another, at Khon Kaen, was wrecked. Violence was also reported in the northern city of Chiang Mai.
In Bangkok, the main rally stage area was empty after the protest leaders' surrender.
One protest leader, Nattawut Saikua, appealed to his supporters to "please return home".
The leaders, labelled terrorists by the army, earlier urged supporters not to give up the fight for political change.
Minutes after they surrendered, Reuters reported that three grenades had exploded outside the main protest site, badly wounding two soldiers and a foreign journalist.
Defiant protesters attacked shops and property and masked men could be seen apparently looting.
With the extent of the continuing resistance unclear, Kavee Chukitsakem, head of research at Kasikorn Securities, told Reuters: "After the red-shirt leaders surrendered, things were out of control.
"It's like insects flying around from one place to another, causing irritation."
The US state department said it deplored the violence and urged restraint on both sides.
Spokesman Gordon Duguid said the US was encouraged that protest leaders had surrendered and called for their supporters to return home.
EU parliamentary president Jerzy Buzek said national reconciliation was now "not simply an option, it is absolutely mandatory".
"Too much blood has been spilled on the streets of Bangkok," he added.
Government spokesman Panitan Wattanayagorn told the BBC the government wanted to reach a peaceful resolution with the protesters but pockets of resistance remained and had to be dealt with.
An Italian photojournalist was killed, while three other reporters, a Dutch person, an American and a Canadian, were among scores of people injured.
According to eyewitnesses, the bodies of six people killed in the violence are lying in a Buddhist temple, AP reported.
If confirmed, this would bring the overall death toll to 12.
Warnings delivered through loudspeakers heralded Wednesday morning's military assault, which saw soldiers in armoured vehicles smashing through the bamboo-and-tyre barricades.
Troops armed with M16 rifles stormed the Lumpini Park area, where demonstrators fled, leaving scattered shoes, overturned chairs and hanging laundry.
The red-shirts had been protesting in Bangkok since 14 March, occupying the shopping district, forcing hotels and shops to close.
But events took a deadly turn last week when the government moved to seal off the area and a renegade general who backed the protests was shot dead.
The red-shirts are a loose coalition of left-wing activists, democracy campaigners and mainly rural supporters of Mr Thaksin.
They are demanding fresh polls because they say the government - which came to power through a parliamentary deal rather than an election - is illegitimate.