The protesters say they will not leave until the PM resigns
Flights from Thailand's international airport have been suspended after hundreds of anti-government protesters stormed the building outside Bangkok.
The demonstrators are in full control of Suvarnabhumi airport, leaving at least 3,000 passengers stranded.
A BBC correspondent says it is the most dramatic move so far in the protesters' campaign to oust the government.
The government is to hold an emergency cabinet meeting, and the head of the army is due to make a statement.
There is speculation that the army chief may impose emergency rule.
A leader of the protesters has rejected a government offer of talks to end the stand-off.
The head of the anti-government People's Alliance for Democracy (PAD), Sondhi Limthongul, said his group would only agree to talks if Prime Minister Somchai Wongsawat resigned.
A series of small explosions among the PAD protestors on Wednesday morning injured several people, underlining the risk of more violent clashes with pro-government groups, says the BBC's Jonathan Head in Bangkok.
Yellow-shirted protestors from the PAD took over strategic areas of the airport, such as the control tower, on Tuesday.
The protesters, who have been occupying a government compound in the capital, claim that the government is corrupt and hostile to the monarchy.
They also accused it of being a proxy for former premier Thaksin Shinawatra, who was ousted in a 2006 coup, but who critics say is still very influential.
Prime Minister Somchai Wongsawat is Mr Thaksin's brother-in-law.
Building the new airport was former PM Thaksin Shinawatra's pet project
Plagued by delays, it opened in 2006, days after Mr Thaksin was overthrown in a military coup
Shoddy construction work was used by the military as one of the justifications for the coup
Designed by German architect Helmut Jahn, thought to have the world's tallest control tower at 132.2m (433.7ft)
The PAD is a loose grouping of royalists, businessmen and the urban middle-class opposed to Mr Thaksin.
The protesters had hoped to intercept Mr Somchai as he returned from an Asia-Pacific summit in Peru, but his flight has been diverted elsewhere.
Now the PAD says it will keep the airport closed until Mr Somchai resigns.
"We will stay until the government steps down. This government is not legitimate," retired university lecturer Sunthorn Kaewlai told the Reuters news agency.
The PAD also handed out leaflets to stranded passengers, apologising to tourists for the disruption, adding that "the alliance believes the measure is crucial to bring an end to the traitorous killer government".
Reports say the authorities have begun evacuating stranded passengers.
However, the evacuation appeared chaotic, with the authorities making no announcement, the Agence France Presse news agency reported.
Christopher Persson, from Sweden, spent the night underneath a check-in desk.
"I understand the people but the airlines are terrible. They've given us no information," he told the Reuters news agency.
Thousands of other passengers spent the night sprawled across suitcases, luggage carts and even security conveyor belts.
Airport director Serirat Prasutanon said operations had been "totally shut down" since early on Wednesday, and that 78 outbound and incoming flights had been affected.
"We are trying to negotiate with them to allow outgoing passengers stranded by the protest to fly," he was quoted by the Associated Press as saying.
Stranded tourists: 'We want to go home'
"The incident has damaged Thailand's reputation and its economy beyond repair."
Organisers say the protest is a "final battle" to bring down the government.
Our correspondent says that the government appears to have followed a strategy of allowing the PAD to attack government buildings while avoiding clashes, in the hope that it will wear the protesters down.
The government has so far resisted calling in the army. Analysts says it is a thinly disguised aim of the PAD to provoke such a move.
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