Rioting and fires swept Bangkok on Wednesday after troops stormed a protest encampment, forcing anti-government protest leaders to surrender but triggering clashes that killed at least six and led to unrest in the north, Reuters reported.
The government extended an overnight curfew in Bangkok to 24 provinces -- nearly one-third of the total. Unrest spread to seven provinces, and town halls were burnt in three northern areas, strongholds of anti-government activists.
"Red shirt" protesters earlier set at least 27 buildings ablaze in the capital, including the Thai stock exchange and at least 16 bank branches. Central World, southeast Asia's second-biggest department store complex, was gutted by fire and looked as if it might collapse, a Reuters witness said.
The unrest is the "most widespread and most uncontrollable" political violence Thailand has ever seen, said political historian Charnvit Kasetsiri. It is exactly 18 years since a major bout of unrest known as "Black May".
"I am confident and determined to end the problems and return the country to peace and order once again," Prime Minister Abhisit Vejjajiva said in a televised address on Wednesday night.
In Washington, a U.S. State Department official told reporters talks were planned with Thai diplomats and officials.
"There has been substantial burning, substantial looting throughout the city and there are reports of sporadic incidents throughout the country as a whole," said Kurt Campbell, Assistant Secretary of State for East Asia and the Pacific.
Britain altered an advisory, recommending against all travel to Bangkok "in view of the highly uncertain security situation and the currently unpredictable violence".
It was unclear whether the continued rioting, after the surrender of the protest leaders, was a final flurry of anti-government action or the start of more intense, widespread fighting.
"The situation is worse than expected now and it's very difficult to stop," said Kavee Chukitsakem, head of research at Kasikorn Securities. "After the red shirt leaders surrendered, things were out of control. It's like insects flying around from one place to another, causing irritation."