Police are being overrun by much larger numbers of PAD guards.
Thai protesters have attacked a second checkpoint near Bangkok's main airport, forcing police forces to retreat.
The confrontation came after police surrounded and cordoned off Suvarnabhumi international airport to block more protesters from arriving.
Authorities are trying to end a blockade of Bangkok's two main airports that has entered its fifth day.
But the protesters say they will not leave until PM Somchai Wongsawat resigns, which he has refused to do.
The BBC's Jonathan Head says it is not clear why the police are so reluctant to end the sit-in, which is costing Thailand's economy dear.
But he says the challenge of storming such a vast building - filled with people of all ages - without causing significant casualties may be beyond the police. And the army is still refusing to help.
The government says it will now coordinate the evacuation of tens of thousands of stranded tourists with foreign embassies in Bangkok.
But with the prime minister still too nervous to come back to the capital from the north of the country, the government's authority to do anything is quickly evaporating.
State of emergency
A state of emergency has been declared at both Suvarnabhumi and the smaller, domestic Don Mueang airport, which the anti-government People's Alliance for Democracy (PAD) have taken over.
PAD protesters have vowed to "fight to the death"
Correspondents say there is an almost triumphant mood among the crowds of people thronging the giant campsite that Bangkok's spectacular airport has now become.
The mood was more jittery this morning as reports came through of thousands of police moving into position to retake the terminal building from the PAD protesters. But the attack never came.
Instead a small contingent of police setup roadblocks at the airports, where at least 2,000 protesters have dug in behind barricades and barbed wire.
Protest leader Sondhi Limthongkul, told supporters he would not negotiate despite a personal plea from the prime minister to hold talks.
Thousands of travellers remain stranded in Bangkok as a result of the blockade, which has already cost the country's economy $4bn (£2.6bn).
Airlines started flying passengers out of a naval airbase 150km (90 miles) from the city on Friday but reports suggested that confusion and delays continued.
Mr Somchai will remain in Chiang Mai indefinitely for his own safety
PM Somchai pledged Friday to use peaceful means to end the siege of the capital's airports and demoted the country's chief of police to what officials said was an "inactive post" in the prime minister's office.
No official reason was given for Gen Patcharawat Wongsuwanbut's demotion, but government spokesman Nattawut Saikuar suggested to Thai TV that it was in connection with the protest crisis.
Mr Nattawut said police had been instructed to "do whatever is necessary to open the airports on the basis of non-violence".
Meanwhile, the government said that Mr Somchai would remain in the northern city of Chiang Mai indefinitely for his own safety.
The BBC's Quentin Sommerville in Bangkok says that Mr Somchai has already lost the confidence of his army chief, Gen Anupong Paochinda, and rumours of a coup are circling in the capital.
Months of turmoil
Thailand has been in political turmoil since former Prime Minister Thaksin Shinawatra was ousted in a military coup in 2006.
Protests continue at Thai airport
The PAD - a loose alliance of royalists, businessmen and the urban middle class - claim that the government is corrupt and hostile to the monarchy.
They have occupied a government complex in Bangkok for months, and declared at the beginning of this week that they were embarking on the "final battle" of their campaign to unseat the government.
Correspondents say the airport protesters appear increasingly isolated and are losing the support of their traditional sympathisers, the business elite.
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