The airport takeover was one of the boldest gambles yet by the People's Alliance for Democracy in its four-month campaign to topple Prime Minister Somchai Wongsawat, whom it accuses of being the puppet of a disgraced fugitive predecessor, billionaire Thaksin Shinawatra.

The alliance vowed to bring its campaign to a final showdown this week, and violence has spiked, including streets clashes between supporters and opponents of the government yesterday that included the first open use of firearms by the anti-government protesters.

Police said 11 government supporters were injured, some with gunshot wounds.

Early on Wednesday, assailants threw four explosives at anti-government protesters, including one targeting a group about a half-mile from Bangkok's main international Suvarnabhumi Airport.

A second was tossed into a crowd of supporters gathered at the domestic Don Muang airport, injuring three others, police said. Two other explosives were thrown in Bangkok but no one was injured.

Demonstrators, some masked and armed with metal rods, had swarmed the international airport overnight, breaking through police lines and spilling into the passenger terminal.

The airport was fully shut down early Wednesday, with incoming flights being diverted to other points in Thailand including Chiang Mai and Phuket, and as many as 500 passengers remained stranded, airport director Serirat Prasutanont said.

"We have to close the airport because of (alliance) protesters blocked the entire airport," Serirat said.

"However, we are trying to negotiate them to allow outgoing passengers stranded by the protest to fly," he said. "The incident has damaged Thailand's reputation and its economy beyond repair."

The alliance said the airport would be shut down until Somchai quits. The prime minister was scheduled to return late on Wednesday from an Asia-Pacific summit in Peru and would land at a military airport, officials said.

The protesters appeared intent on forcing the military to intervene and bring down the elected regime. Army commander Gen. Anupong Paochinda has repeatedly ruled out a coup, though he has also said the army "will keep peace and order to protect the public and uphold important institutions like the monarchy."

The alliance has staged a number of dramatic actions in recent months. It took over the prime minister's office in late August and twice blockaded Parliament -- one time setting off street battles with police that ended with two people dead and hundreds injured.

Support for the alliance has been waning in recent weeks, and the group appeared to be edging toward bigger confrontations involving fewer though more aggressive followers in hopes of creating chaos.

"Now, they are openly creating instability and provoking a military coup," said Thitinan Pongsidhirak, a political scientist at Bangkok's Chulalongkorn University.