Government spokesman Nattawut Sai-kau said Prime Minister Somchai Wongsawat and his six-party coalition would step down. Somchai, who has been working from the northern city of Chiang Mai since Wednesday, accepted the ruling with equanimity.
"It is not a problem. I was not working for myself. Now I will be a full-time citizen," he told reporters in Chiang Mai.
Protest leaders said they would end their airport protests -- which have stranded hundreds of thousands of travelers -- on Wednesday.
Officials at Bangkok's Suvarnabhumi international airport said some passenger flights would resume on Friday. The airport reopened to cargo flights Tuesday.
Members of the People's Alliance for Democracy, occupiers of the international airport, cheered and hugged after they heard news of the ruling, which was read from a protest stage outside the main terminal.
"My heart is happy. My friends are very happy," said Pailin Jampapong, a 41-year-old Bangkok housekeeper choking back tears as she jumped up and down.
"This is a blow for corruption," said Nong Sugrawut, a 55-year-old businessman at Suvarnabhumi.
Somchai had become increasingly isolated in recent weeks. Neither the army, a key player in Thai politics, nor the country's much revered king had offered him firm backing.
But hundreds of his supporters gathered outside the court to express their anger, saying the swiftness of the ruling -- which came just an hour after the closing arguments ended -- reeked of predetermination. At one point they cut off the power supply to the court, but electricity was restored with diesel generators.
"The court is not qualified to make this ruling. They are nothing more than apologists for the alliance, who are ruining the country," an activist shouted through a megaphone outside the court.
Somchai's People's Power Party, the Machima Thipatai party and the Chart Thai party were found guilty of committing fraud in the December 2007 elections that brought the coalition to power with a thumping majority.
The case stems from an earlier Supreme Court conviction of a PPP executive committee member, Yongyuth Tiyapairat, who was found guilty of buying votes. Under Thai law, an entire party can be disbanded if one executive member is found guilty of electoral fraud. Similar individual cases brought down the other parties.
The court dissolved the parties "to set a political standard and an example," said Court President Chat Chalavorn. "Dishonest political parties undermine Thailand's democratic system."
The ruling sends Somchai and 59 executives of the three parties into political exile and bars them from politics for five years. Of the 59, 24 are lawmakers who will also have to resign their parliamentary seats.
But lawmakers of the three dissolved parties who escaped the ban can join other parties, try to cobble together a new coalition then choose a new prime minister.
Until then, Deputy Prime Minister Chaowarat Chandeerakul will become the caretaker prime minister, said Suparak Nakboonnam, a government spokeswoman. She said parliament will have to pick a new prime minister within 30 days.
Despite the appearance of a smooth political transition, the ruling is expected to widen the dangerous rift in Thai society that many fear could lead to violence between pro- and anti-government groups.
Late Monday, an explosive device fired from an elevated highway fell among hundreds of protesters inside Don Muang airport, killing one person and wounding 22. The death raised to seven the number of people killed in bomb attacks, clashes with police and street battles between government opponents and supporters.
Protest leaders, meanwhile, told alliance members to stay put at Suvarnabhumi and Don Muang domestic airports, despite the ruling. Alliance leaders will meet later Tuesday to decide the next course of action, said Sirichai Maingam, an alliance leader.
Up to 10,000 protesters have beseiged the airports, forcing authorities to shut them down, cutting off all commercial air traffic to the capital, stranding more than 300,000 foreign travelers and bleeding millions of dollars from the country's economy in lost business and tourism.
With the two main airports closed, stranded travelers are being flown out of provincial airports with limited passenger capacity or are making their way overland to neighboring Malaysia.
Even if the protesters disperse, officials have said it will take at least another week before the airports become operational again.
Chaisak Anksuwan, general director of the Department of Aviation, said he has authorized cargo flights to land at the airport Tuesday morning and that the first flights were expected to arrive later in the day. He said the development was unrelated to the court's ruling.
The protesters accuse Somchai of being a proxy of ousted Prime Minister Thaksin Shinawatra, the alliance's original target. Thaksin, who is Somchai's brother-in-law, was deposed in a 2006 military coup and has fled the country to escape corruption charges.
Alliance supporters are largely middle-class citizens who say Thailand's electoral system is susceptible to vote-buying and argue that the rural majority -- the Thaksin camp's political base -- is not sophisticated enough to cast ballots responsibly.
They have proposed discarding direct elections in favor of appointing most legislators, fostering resentment among rural voters.
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