Forty-nine people were reported hurt in the first serious clash between the two sides in ongoing protests that have roiled this southeast Asian nation and came a day after the country's ousted prime minister called for a revolution.
CBS News correspondent Celia Hatton reports that the violcence is the latest round in a political tug-of-war between the "red shirts," who support the former Prime Minister Thaksin Shinawatra, and the "yellow shirts," who back the current government. The reds are mostly from Thailand's poor majority; the yellows come from the country's urban middle classes, Hatton reports.
While the government has declared a state of emergency, protesters controlled many streets in the capital Bangkok. They had earlier commandeered public buses and swarmed triumphantly over military vehicles in defiance.
In the starkest example of the chaos, a mob of the red-shirted protesters smashed cars carrying Prime Minister Abhisit Vejjajiva and his aides.
The clash began between 4 a.m. and 5 a.m. Monday, as troops in full combat gear lined up and advanced to disperse the protesters, according to witnesses and television footage.
The soldiers fired hundreds of rounds from their M-16 automatic rifles as they advanced, though it was unclear whether they were firing at or over the protesters. Some witnesses said tear gas was also fired.
The official Erawan emergency coordination center said 49 people were injured on both sides and taken to hospitals.
Protesters set fires that were still burning 1-1/2 hours later and retreated into side streets near the Din Daeng intersection, where there is an on-ramp to the main expressway leading north from the capital.
The clash appeared to be an isolated one, taking place several miles away from the main encampment of thousands of protesters outside the prime minister's offices.
Police Gen. Vichai Sangparpai said up to 30,000 demonstrators were scattered around the city. Police vans at some intersections were abandoned and looted. Protesters used buses to barricade several major roads.
Thaksin, regarded by most of the protesters as their leader, called for a revolution and said he might return from exile to lead it.
Thaksin fled the country last year, before a court convicted him in absentia of violating a conflict of interest law.
"Now that they have tanks on the streets, it is time for the people to come out in revolution. And when it is necessary, I will come back to the country," he said in a telephoned message to followers outside Abhisit's office.
The message was broadcast over a video link projected on giant screens and relayed on supporters' Internet sites.
Political tensions have simmered since Thaksin was ousted by a military coup in 2006 for alleged corruption and abuse of power. He remains popular in the impoverished countryside for his populist policies.
His opponents - many in urban areas - took to the streets last year to help bring down two pro-Thaksin governments, seizing Bangkok's two airports in November for about a week.
The emergency decree bans gatherings of more than five people, forbids news reports that threaten public order and allows the government to call up military troops to quell unrest.
Army spokesman Col. Sansern Kaewkamnerd said soldiers and police were being moved to more than 50 key points in the city, including bus and railway stations.
He said the military presence was not a sign of an imminent coup - a common feature of Thai political history.
Abhisit, speaking in a nationally televised address just before midnight, called on the public not to panic and to cooperate to end the crisis.
"In the next three to four days, the government will keep working to return peace and order to the country," he said.
Protests were also reported in several provinces of northern and northeastern Thailand. The protests could prompt the military to intervene - a high possibility in a country that has experienced 18 military coups since the 1930s.
In an apparent attempt to dispel speculation of a breakdown in the security chain of command, Abhisit was flanked by Cabinet members and top military and police officers during the address.
"They have tried to spread the rumor that there is no unity in the operation of (security) officials," he said. "We will try to continuously communicate correct information to the public."
His government suffered a major humiliation Saturday when it failed to stop hundreds of demonstrators from storming the venue of a 16-nation Asian summit, forcing its cancellation and the evacuation of the leaders by helicopter and boat.
There were signs Sunday that the government might again not be able to contain the protesters.
Demonstrators swarmed over two armored personnel carriers outside a luxury shopping mall, waving flags in celebration. An old lady atop one of the vehicles screamed "Democracy!"
Outside the Interior Ministry, a mob attacked Abhisit's car with poles, a ladder and flower pots as it slowly made its escape. At least six people were injured, including two security guards for Abhisit. Police in riot gear did nothing.
"The government can't do anything," said Lada Yingmanee, a 37-year-old protester. "We will show them what tens of thousands of unarmed civilians can do. The people will finally rule our beloved Thailand."
Demonstrators from the United Front for Democracy Against Dictatorship say Abhisit's four-month-old government took power illegitimately and want new elections. They also accuse the country's elite - the military, judiciary and other unelected officials - of undermining democracy by interfering in politics.
Parliament appointed Abhisit in December after a court ordered the removal of the previous pro-Thaksin government for election fraud, sparking Thaksin supporters to take to the streets. Their numbers grew to 100,000 in Bangkok last week.