Hundreds of anti-government protesters forced several dozen Thai riot police to abandon a checkpoint on Saturday as they tightened their siege of the country's main airport, witnesses said.
About 2,000 anti-government protesters forced back about 150 police from 1km north of Suvarnabhumi Airport, although the incident passed off without violence.
A state of emergency has been declared at Suvarnabhumi international airport and the smaller Don Mueang airport, which anti-government group People's Alliance for Democracy (PAD) have taken over.
At Suvarnabhumi hundreds of riot police have moved into place. Police have not yet moved to break up the protests but have said they are prepared to take "other steps" if negotiations failed.
Both airports have been paralysed by the protests and thousands of passengers are stranded in Bangkok, including hundreds of Thai Muslims trying to travel to Saudi Arabia for the Hajj pilgrimage.
The People's Alliance for Democracy (PAD) say they will not leave until Prime Minister Somchai Wongsawat resigns, which he has refused to do.
The protesters are seeking to oust Mr Wongsawat in the latest escalation in a long-running political crisis. The loose alliance of royalists, businessmen and the urban middle class claims the government is corrupt and hostile to the monarchy.
A day after their police chief was sacked for mishandling the protests, commanders on the ground said they would not yet try to evict by force the thousands of protesters at Suvarnabhumi and Don Muang airports.
But PAD supporters were taking no chances and on Saturday deflated the tyres of ambulances and police vehicles at the police checkpoint. Several vehicles were left stranded in the middle of the road.
The PAD say they are ready for a prolonged siege, with their "security guards" armed with clubs, sticks and golf clubs, and dug in behind a series of barricades of fire trucks, razor wire, car tyres and luggage trolleys.
In a televised address on Thursday night, Mr Somchai said the PAD members barricaded at the airports were doing massive damage to the economy, but he would avoid violence to end the protests.
The airport sit-ins have forced hundreds of flights to be cancelled, stranded thousands of foreign tourists and grounded millions of dollars of air cargo.
The government began shuttling thousands of stranded tourists by bus to U-Tapao, a Vietnam War-era naval airbase 150km east of Bangkok, as an alternative landing site for airlines, but travellers reported delays and confusion.
The BBC reports the airport closures will cost the country about $US4 billion in lost business and cause serious damage to its reputation as a tourist destination.
Police chief demoted
The moves follow news of the demotion of the country's chief of police.
No official reason was given for General Patcharawat Wongsuwanbut's demotion, but government spokesman Nattawut Saikuar suggested to Thai television that it was in connection with the protest crisis.
Major General Prateep Tanprasert has been appointed acting police chief.
Meanwhile, the government has said that Mr Somchai will remain in the northern city of Chiang Mai indefinitely for his own safety.
The BBC reports Mr Somchai has already lost the confidence of his army chief, General Anupong Paochinda, and rumours of a coup are circling in the capital.
Earlier this week, General Anupong urged Mr Somchai to call a snap election as a way of easing the crisis.
Thailand has been in political turmoil since former Prime Minister Thaksin Shinawatra was ousted in a military coup in 2006.