Officials warned that security forces had been authorised to shoot looters and arsonists.
|Abhisit said he was determined to end the conflict with the red shirts [AFP]
"The end of the rally has dissatisfied some protesters, especially those who are armed," Abhisit said.
"So they created trouble, particularly arson in some areas."
The fires were started after security forces stormed the barricades around the red shirts' sprawling protest camp in central Bangkok, forcing hundreds of demonstrators to retreat.
The overnight curfew, which also covered 24 provinces outside the Thai capital, was lifted at dawn on Thursday with Bangkok reported to be under a tense calm as residents counted the cost of the latest clashes.
Six key red shirt leaders surrendered to police following Wednesday's assault, while officials said several other protest leaders had escaped and were being hunted by security forces.
At least six people were killed in the offensive, including a freelance Italian photojournalist, with dozens of others injured.
Among the buildings set on fire as the red shirts retreated from their protest camp was the Bangkok stock exchange building and the Central World mall, the second largest shopping centre in Southeast Asia.
The offices of state-run Channel 3 television were also set ablaze, forcing the evacuation of its executives by helicopter. Police rescued the rest of the staff.
The English-language Nation and Bangkok Post newspapers evacuated their staff after threats from the red shirts while a large office building down the street from the Bangkok Post office was set alight.
Unrest also spread to Thailand's rural north and northeast, areas that are seen as strongholds of red shirt support.
Local media reported protesters set fire to government offices in Udon Thani and vandalised a city hall in Khon Kaen.
Udon Thani's governor asked the military to intervene. TV images showed troops retreating after being attacked by mobs in Ubon Ratchathani.
Tony Birtley, Al Jazeera's correspondent in Bangkok, said violence had spread to some extent.
"There have been outbursts of violence in several places in the north and northeast," he said.
"A contact of mine phoned me from Chiang Mai [in the north] saying that fire engines were set alight, property was destroyed and barricades were set up there, so the violence is spreading."
Against this backdrop, Thaksin Shinawatra, the country's ousted prime minister whom many of the red shirts support, said he feared a military crackdown could lead to guerrilla warfare across the country.
|Officials warned that the army had orders to shoot looters or arsonists on sight [AFP]
"There is a theory saying a military crackdown can spread resentment and these resentful people will become guerrillas," he told the Reuters news agency by telephone.
Thaksin, who is accused by the government of bankrolling the protests and inciting unrest, denied he had undermined peace talks, saying he was not the "mastermind of the terrorists".
But Tim Forsyth, a South Asia expert based in the UK, cast doubt on Thaksin's claims.
"Unlike previous demonstrations involving the red shirts, this time they have had a small proportion of highly armed specialised paramilitary people amongst them," he told Al Jazeera.
"They have also had a very clear intention of trying to embarrass and bring down the current government.
"This suggests that Thaksin may well be behind all this because after all he wants his money unfrozen and he wants to influence Thai politics again."
Wednesday's crackdown began with about 100 soldiers armed with automatic rifles and shotguns, along with several machinegun-mounted armoured personnel carriers, breaching the red shirts' barricade at the southern end of their protest site in Bangkok's Rachaprasong neighbourhood.
The armoured vehicles repeatedly rammed the barricade, made up largely of tyres, sharpened bamboo poles and razor wire, before breaking through the flattened structure.
Al Jazeera's Tony Birtley said the raid on the protest camp was a relative success for the military.
"This was one of those feared operations that people believed that if the military came in heavy handed, hundreds of people could be killed and injured," he said.
"That hasn't happened. However, there are many problems and grievances that exist for these people and that will have to be addressed by any Thai government in the future."