"I do not have any intention to hurt any members of the public," he said.
But the emergency law, which suspends civil liberties of people in the airport - including free assembly and freedom of movement - could pave the way for clashes.
The smaller Don Muang airport, which until 2006 was Bangkok's main airport, was closed to all air traffic after about 3,000 protesters from the People's Alliance for Democracy (PAD) gathered outside, officials said.
Al Jazeera's Hannah Belcher, reporting from the main Suvarnabhumi airport which has also been overrun by protesters, said that while the area was calm, several PAD supporters were carrying makeshift weapons and the situation had the potential to become explosive.
"The two airports that serve Bangkok are completely closed"
Saererat Prasutanond, Airports of Thailand
Prior to the declaration of the state of emergency, the European Union expressed its concern at the situation, warning that an "anti-constitutional attempt to interfere in the democratic process would have a negative impact" on relations.
The statement is a possible warning to Thailand's powerful armed forces, amid speculation that the military could intervene in the crisis and move against the government.
Somchai has urged the army to stay in its barracks amid rumours of an imminent coup, Nattawut Saikuar, a government spokesman, said.
Nattawut also denied rumours that Somchai planned to sack Anupong Paochina, the army chief, a day after the general called for a snap election to defuse the country's political crisis.
The closure of the Don Muang airport means that Bangkok, a major Asian international air hub, is without an operating civilian airport.
The military has until now remained neutral in the face-off between the government and the PAD.
Somchai and his cabinet have been operating out of temporary offices at Don Muang since August, when PAD protesters occupied the grounds of Government House in central Bangkok.
|Key facts: The PAD
Group is a loose coalition of royalists, businessmen and urban middle class -Thailand's traditional elite.
PAD led protests that triggered 2006 coup against the then PM, Thaksin Shinawatra.
Supporters wear yellow shirts, a colour associated with Thailand's revered king.
Group accuses Thaksin supporters of pushing to turn Thailand into a republic, an allegation rejected by Thaksin.
Critics say PAD's contempt for results of three democratic elections show it is neither popular nor democratic.
Click here for more on the PAD
Don Muang airport normally serves a handful of domestic flights and budget airlines but had been used as a back-up after thousands of PAD supporters stormed the main Suvarnabhumi international airport on Wednesday, forcing it to close and stranding thousands of travellers.
The PAD protesters said they were blockading Don Muang airport to prevent cabinet ministers flying up to Chiang Mai for a meeting.
Suvarnabhumi remained closed on Thursday morning with PAD supporters camped out in the airport terminal and little sign of them preparing to move.
The airport will remain closed until at least 6pm [11:00 GMT] on Saturday, the airport authority said.
The latest closure intensifies the PAD's stranglehold on the Thai capital, as they step up what they say is their "final battle" to force the government from power.
Three of Thailand's neighbours raised the prospect of postponing a regional leaders meeting in the country's capital next month, but the government said it remained ready to host the Association of South-East Asian Nations (Asean) summit from December 15-18.
Vietnam, Cambodia and Laos said on Thursday they would ask Asean whether the summit, which has already been moved from Bangkok to the northern city of Chiang Mai, should be postponed.
Shockwaves from the protests are reverberating around the country, with one PAD protester reportedly shot dead on Wednesday in Chiang Mai by pro-government supporters.
Thailand's political standoff began three months ago when the PAD occupied the prime minister's office compound and has paralysed the government, battered the stock market and dealt a serious blow to the country's tourism industry.