Somchai accused the protesters of "holding the country hostage and the public hostage."
"I do not have any intention to hurt any members of the public," he added, though the imposition of the measures raised the possibility that violent clashes could break out as authorities moved on Suvarnabhumi international airport and the city's older, smaller Don Muang airport.
The declaration, which applies only to the two terminals, empowers the government to suspend some civil liberties, including restricting the movement of people and prohibiting mass assembly in certain locations.
The People's Alliance for Democracy, which has been demanding the resignation of Somchai and his government, seized control of Suvarnabhumi international airport on Tuesday, forcing the cancellation of all flights in and out of the capital and sending thousands of tourists to hole up in Bangkok hotels.
The standoff, which began three months ago when the group occupied the prime minister's office compound, has paralyzed the government, battered the stock market, spooked foreign investors and dealt a serious blow to the tourism industry.
Some Thai economists have estimated that tourism losses alone in the remainder of this year could run to 150 billion baht ($4.2 billion), equal to 1.5 percent of gross domestic product.
Government Spokesman Nattawut Sai-kua earlier called the seizure of the airports "a terrorist act."
"The prime minister says we have to use peaceful means," he said. "(Security officials) will negotiate first and we will go step by step, adhering to international standard and the law."
Somchai said navy personnel would help police in clearing Suvarnabhumi international airport. The air force will assist at Don Muang. He did not say when the authorities would act.
Amid the standoff, coup rumors swirled around the capital after Thailand's powerful army commander Gen. Anupong Paochinda suggested Wednesday that Somchai call new elections, and the prime minister rejected the idea. The rumors were further fueled by press reports of tank movements that the military later said were only a training exercise.
"The government is asking all troops to remain in position and not to move anywhere in order to quell a rumor" of a coup, said Nattawut. "We ask the public not to panic or to worry about the situation."
Army spokesman Col. Sansern Khaewkamnerd also denied the rumor.
"As the army chief has said many times, a coup will not resolve anything, and there is no planning going on to stage one," he told The Associated Press.
In September 2006, the military staged a bloodless coup to oust former Prime Minister Thaksin Shinawatra after months of protests staged by the same group that is demanding the resignation of the current government. The protest alliance says the prime minister, who is Thaksin's brother-in-law, is merely the former leader's puppet.
They accuse Thaksin and his allies of corruption and abuse of power. Thaksin is in exile, a fugitive from a conviction for violating a conflict of interest law.
Despite assurances that there would be no coup this time, government supporters called on their faithful to resist any military takeover attempt against Somchai.
Vipu Talang, a leading pro-government activist, said Somchai's supporters were moving to gather at a large open field in Bangkok. "We will not accept, and the people will resist, any attempt by the army to take power," he said.
The Nation newspaper reported on its Web site that several businesses in the capital sent their employees home early in anticipation of a possible coup, which could be accompanied by a curfew and other limitations on movement. Several government offices also closed early.
Thailand's long-running political crisis began to boil over Tuesday when the protest group took over Suvarnabhumi, just outside Bangkok, forcing virtually all flights to be canceled and stranding 4,000 travelers.
At the same time, a number of violent confrontations erupted between government supporters and opponents. At least one person was killed, according to police.
The government is drawing up plans to begin flying out thousands of tourists with "urgent needs" from one or two military bases in the next 48 hours. That could include parents with young children and people with medical conditions, said Weerasak Kowsurat, Thailand's tourism minister.
They would be flown on Thai Airways flights to Singapore or Malaysia. The planes could then return with incoming passengers, Weerasak said. The government also may use buses and trains to transport tourists to other airports in Thailand.
A Thai Airways flight from Los Angeles landed Thursday at U-Tapao air force base, 140 kilometers (90 miles) southeast of Bangkok, the airline said.