By seizing the airports, including two-year-old Suvarnabhumi Airport, which handles about 40 million passengers a year, the demonstrators have put a virtual stranglehold on the country's crucial tourism industry.
The siege is causing so much damage to Thailand's economy that it must be resolved "within days, not weeks," Suchit Bunbongkarn, director of the Institute of Security and International Studies at Chulalongkorn University, said from Bangkok.
"There are more than 10,000 protesters, and it's not very easy for anybody to clear them out," he said. "Damage would be done to the airport, let alone the lives of the people there."
The alliance draws its main support from Thailand's cities and says Somchai is a corrupt puppet of his brother-in-law, Thaksin Shinawatra, who was ousted as prime minister by a bloodless coup in 2006.
"The best solution is probably for the government to resign first and then we have to do something else later," Suchit said. "I would say, at the moment, the government hasn't been functioning at all."
The National Assembly voted Somchai into office in September after the Constitutional Court ruled that Prime Minister Samak Sundaravej had to resign because he had violated conflict-of-interest laws by appearing as a guest host on two televised cooking shows while in office.
Tourism is Thailand's top source of foreign exchange, earning it $16 billion a year. Its biggest airline, Thai Airways, said Tuesday that it expected to lose $14 million a day because of flight cancellations.
The tourism industry has suffered two sharp declines in the past six years, first during the 2002-2003 SARS crisis, when the virulent spread of potentially deadly severe acute respiratory syndrome scared off many travelers.
Then the Indian Ocean tsunami struck in 2004, devastating some of Thailand's most popular resort areas during the peak Christmas season. The tsunami claimed more than 220,000 lives, 5,400 of them in Thailand.
The Associated Press contributed to this article.
