He also asked thousands of protesters to end their siege of the main international Suvarnabhumi airport. It has been shut since Tuesday night, leaving hundreds of flights canceled and drawing world attention to a turmoil that has reduced Thailand to a dysfunctional nation.
The anti-government protests, which gathered pace four months ago, have paralyzed the government, battered the stock market, spooked foreign investors and dealt a serious blow to the tourism industry.
The crisis worsened early Thursday as authorities shut down the Don Muang domestic airport, which had been receiving some diverted flights from Suvarnabhumi.
Serirat Prasutanont, chief of Thailand Airport Authority, said authorities feared that protesters who stormed the Don Muang terminal building late Wednesday might harm passengers and aircraft.
He said authorities might consider using the U Ta Pao air force base, 140 kilometers (90 miles) southeast of Bangkok, and were alerting airports nationwide to be ready to receive more diverted flights.
The closure of the two airports left thousands of foreign tourists stranded, including Americans trying to get home for their Thanksgiving holiday Thursday.
Cheryl Turner, 63, of Scottsdale, Arizona, had asked neighbors to pull an 18-pound turkey from her freezer a day ahead of time to defrost so she could cook it for a traditional Thanksgiving dinner.
"My turkey is sitting in the sink at home," she said.
Inside the terminal, many passengers were seen lounging on the floor. One Westerner created a commotion, yelling: "I need to get to Phuket now!"
As a female official tried to calm him, he shouted: "You will help me? I can't go by bus! I can't go by plane! How do I get there!"
The protests are being led by a loose coalition known as the People's Alliance for Democracy. It accuses Somchai of acting as the puppet for former Prime Minister Thaksin Shinawatra, who was ousted in a September 2006 military coup after being accused of corruption and abuse of power. Thaksin, who is Somchai's brother-in-law, is in exile, a fugitive from a conviction for violating a conflict of interest law.
On Wednesday, a district court ordered the alliance leaders and their supporters to immediately leave Suvarnabhumi International Airport, calling it "an infringement on other individuals who have freedom of movement."
However, the protesters are unlikely to heed the order -- a reflection of their boldness amid the government's unwillingness to use force for fear of causing bloodshed.
Still, Prime Minister Somchai, who returned from Peru on Wednesday but was forced to land in the northern city of Chiang Mai, is also not budging from his position.
Somchai said in an address to the nation that he came to power through elections and has "a job to protect democracy for the people of Thailand."
The statement amounted to a rejection of Army Gen. Anupong Paochinda's suggestion to quit, which seemed to put him on a collision course with the military, although the general has said he would not launch a coup.
Somsak Kosaisuk, a key protest alliance leader, said protesters stormed Don Muang airport because they want to prevent members of Somchai's Cabinet from flying to Chiang Mai for a proposed emergency Cabinet meeting Thursday.
The drive from Bangkok to Chiang Mai takes about eight hours.
Still, government spokesman Nattawut Saikau said the emergency meeting would go ahead. "The key issue is how to deal with escalating violence in the country," he told The Associated Press.
The People's Alliance for Democracy insists it would continue its airport occupation and other protest activities until Somchai resigns. It rejected the general's proposal for new elections, pushing instead for the appointment of a temporary government.
On Thursday, the EU and the British Foreign Office expressed concern at the deteriorating situation.
"We urge all sides to this political dispute to resolve their differences peacefully and legally, respecting Thailand's democratic institutions," Foreign Office Minister Bill Rammell said.
The European Union said in a statement that "any anti-constitutional attempt to interfere in the democratic process would have a negative impact on EU/Thailand relations."
As the deadlock continued, political violence spread Wednesday to Chiang Mai, where government supporters attacked a radio station aligned with the protesters. Separately, there were unconfirmed reports that one man was killed and several people assaulted in an attack on the city's local airport.
Thousands of travelers were stranded in Bangkok when members of the alliance swarmed the airport Tuesday night, forcing a halt to virtually all outgoing flights.
The protest alliance launched its current campaign in late August, storming the grounds of the prime minister's office, which they continue to use as their stronghold. The group has also tried twice to blockade Parliament, in one case setting off a daylong street battle with police that left two people dead and hundreds injured.
