Pressure is building on Thai Prime Minister Samak Sundaravej, amid spiralling protests against his rule.
Activists who had been holed up in the main government complex for almost four days spilled out to surround police headquarters before being dispersed.
Outside Bangkok, strikes in support of the protests have disrupted travel.
The head of the army dismissed rumours of a coup, but called on the government to negotiate with protesters, which the authorities have so far refused to do.
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They say Mr Samak is a puppet of former Prime Minister Thaksin Shinawatra, who was ousted by a military coup in 2006 and is now in exile to avoid facing trial over corruption charges.
According to the BBC correspondent in Bangkok, Jonathan Head, the position of Mr Samak, who had hoped he could tough out these protests, now looks precarious.
The prime minister has said he might declare a state of emergency if the situation worsens.
Protests spread
In a matter of hours, the tables have turned against Thailand's embattled prime minister, our correspondent says.
Earlier this week Mr Samak was praised for his restraint in dealing with the PAD, which has been occupying his office compound since Tuesday.
But he suddenly found his position weakened late on Friday when the police pulled back and the triumphant protest leaders were left in control of the complex. See a map of key protest areas in Bangkok
About 2,000 of them then besieged the city's police headquarters.
Police fired what appeared to be tear gas on the crowds, and they dispersed shortly afterwards. One correspondent reported seeing several protesters vomiting.
Demonstrations are also said to have taken place at two major airports in the south of the country.
Phuket and Krabi airports have closed, and there were reports of blockades in Hat Yai. Reports suggested protesters were planning to mount similar actions at Chiang Mai and Chiang Rai in the north.
Some rail workers have gone on strike in support of the protests, and to demand better work conditions, disrupting services on lines from Bangkok to the country's north-east.
And union leaders - including those from flagship airline Thai Airways - have called for workers to join in the stoppage.
Precarious
Rumours of an impending coup have been circulating in Thailand throughout the week, our correspondent says.
For three days Gen Anupong has deliberately kept his forces above the fray.
But on Friday he decided it was time to step in - insisting the military would not mount a coup, but also calling on the government to negotiate with protesters.
"The army will not stage a coup. The political crisis should be resolved by political means," he said, according to AP news agency.
The military's involvement is an indication of the seriousness of the current situation - and correspondents say that some senior military officers are believed to be pressing the prime minister to offer his resignation.
Anti-Thaksin
This crisis began on Tuesday, when thousands of PAD supporters took to the streets of Bangkok and forced their way into government buildings and a state TV station.
Nine PAD leaders now face charges of insurrection for these actions, a crime that could potentially carry the death penalty.
The PAD was originally formed in the months before the 2006 coup, to demand Mr Thaksin's resignation. It has re-emerged now that the country is being led by his former ally Mr Samak.
It has a passionate following in various parts of the country, especially Bangkok, and some powerful backers among the elite, our correspondent says.
But it has little support in most of rural Thailand, which voted strongly for Prime Minister Samak, and Mr Thaksin before him.
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