The statement is a possible warning to Thailand's powerful armed forces, amid speculation the military could intervene in the crisis and move against the government.
Somchai Wongsawat, the prime minister, urged the army on Thursday to stay in its barracks amid rumours of an imminent coup, Nattawut Saikuar, a government spokesman, said.
He also denied rumours that Somchai planned to sack army
chief Anupong Paochina a day after the general called for a
snap election to defuse the country's political crisis.
"The two airports that serve Bangkok are completely closed"
Saererat Prasutanond, Airports of Thailand
Thursday's closure of the Don Muang airport means that Bangkok, a major Asian international air hub, is without an operating civilian airport.
"I authorised Don Muang's director-general to close the airport from midnight. It is closed indefinitely until normalcy is restored," Saererat Prasutanond, president of operator Airports of Thailand, told Thai television.
"The two airports that serve Bangkok are completely closed."
The military has until now remained neutral in the face-off between the government and the PAD.
|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
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.
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.
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.
Meanwhile, the 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.
Somchai himself flew in to Chiang Mai on Wednesday evening, returning to Thailand after last weekend's Asia-Pacific Economic Co-operation (Apec) summit in Peru.
In a televised national address shortly after landing, he insisted his government was legitimate and rejected calls for him to step down.
"I reassure the people that this government, which is legitimate and came from elections, will keep functioning until the end," he said.
Somchai said the cabinet would hold an emergency cabinet meeting on Thursday to discuss "measures" against protesters in Bangkok who he said were trying to subvert the democratic process.
The PAD protesters who later blockaded Don Muang airport said they were doing to so to prevent cabinet ministers flying up to Chiang Mai for the meeting.
Somchai's comments came after Anupong's call for him to dissolve parliament and hold fresh elections.
At a press conference on Wednesday, the army chief said that if he launched a coup, "the problems would be solved once and for all. But there would be a lot of consequences including the international reaction".
|The PAD says it is waging a "final battle" to force the government's hand [AFP]
"We will send him [the prime minister] a letter to inform him that he must dissolve the house and call new elections."
The PAD, a loose coalition of business leaders, urban middle classes and royalists, accuses the government of being tainted by corruption and of being a puppet of Thaksin Shinawatra, the exiled ex-prime minister who was ousted from power in a 2006 coup.
Larry Jagan, a journalist and commentator based in Bangkok, told Al Jazeera that the group's latest moves to escalate their protests seemed aimed at forcing Somchai to take tougher action.
"I think he is going to have to act and the possibility is that he will call a state of emergency, declare martial law, so that the authorities can deal more effectively with the protesters," he said.
"What we're seeing is a battle between the traditional elites who want to return to something like a paternal democracy and the new middle classes under Thaksin, the former prime minister, who really want to open up Thailand and see a global presence for the country."