A police officer was abducted on Saturday after protesters forced riot police to abandon a checkpoint near the airport, witnesses said.
The officer was put in a vehicle and driven away, the Associated Press reported, but no further details about his fate were revealed.
The capital remains cut off from all civilian air traffic on Saturday, stranding thousands of travelers and dealing a severe blow to the economy and tourism industry.
More protesters arrived at the airport in cars and buses despite road blocks set up by police, boosting their number to several thousand.
One of the main elevated roads into the airport is controlled by the alliance's guards who are manning their own checkpoint.
Police, many in full riot gear, also had a much more visible presence, guarding an airport hotel and airport management offices, and assembling in a nearby administrative building.
There appeared to be several hundred in the area.
Riot police with truncheons and shields gathered at Thailand's Suvarnabhumi airport after talks with protesters apparently failed to end the crippling blockade.
Up to 100 police set up a perimeter at the airport on Friday, about 300 metres from where protesters had besieged the main terminal, a Reuters correspondent at the airport said.
Earlier police said they hoped talks with protest leaders would end the siege, but warned they would "take other steps" if they failed.
"We are asking them to allow the airport to resume operations," Lieutenant-General Suchart Muenkaew, the chief police negotiator, said.
"We will keep talking, but if [talks] fail we will take other steps. The last step will be to disperse them."
The government had backed down from its threat to forcibly remove protesters, saying it will try to negotiate with them on the "basis of non-violence".
Al Jazeera's Step Vaessen, reporting from Suvarnabhumi airport, said: "The riot police have arrived. [They are] 200 metres away from the airport and the demonstrators are watching them. It is very unclear what they are going to do. They are in full battle gear."
But the PAD leader had agreed to continue with negotiations and it was "increasingly becoming difficult to remove the protesters", our correspondent said.
Police deployment at the airport coincided with news on Friday that Thailand's police chief had been sacked by Somchai Wongsawat, the prime minister.
No reason was given in the order signed by Somchai moving General Patcharawat Wongsuwan to an inactive post in the prime minister's office.
|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
The siege at the airport has cut the Thai capital's air links to the world, leaving thousands stranded and hurting the tourist-dependent economy.
The University of the Thai Chamber of Commerce said if the political turmoil and airport closures go on for another month, it would cost the economy up to $6 billion.
A government spokesman said the economy could lose at least $2.8 billion if the sieges drag on for a month.
GDP growth for the year could be cut to 4 per cent from a current estimate of 4.5 per cent, already a seven-year low, he said.
Thousands of tourists have been stranded since protesters occupied the airport on Tuesday and the old, smaller Don Muang airport mostly used for domestic flights, on Wednesday.
Thailand's political standoff began three months ago when the PAD occupied the prime minister's office compound.
The protesters accuse Somchai of being a front for Thaksin Shinawatra, who was removed from office as prime minister in September 2006.
Somchai has accused the protesters of holding the country at ransom.
The protesters remained defiant on Friday, vowing to "fight to the death" if police tried to disperse them.
"We are ready to defend ourselves against any government's operations to get us out of those places," Parnthep Wongpuapan, a protest leader, told the Associated Press news agency.
"We are going to stay at the airports until Somchai resigns."
The European Union has voiced concern over the situation in Thailand, warning that an "anti-constitutional attempt to interfere in the democratic process would have a negative impact" on ties, in a clear reference to speculation of a possible military intervention.
A Thai government spokesman said on Thursday the prime minister had urged the army to stay in its barracks amid rumours of an imminent coup.
He also denied rumours that Somchai planned to sack Anupong Paochina, the army chief, a day after the general called for the premier to step down and call a snap election to defuse the country's political crisis.
The military has consistently said it will not carry out another coup after its 2006 move to oust Thaksin Shinawatra proved unsuccessful in solving the country's political woes.