Thailand's prime minister says he would consider declaring a state of emergency if anti-government rioting gets worse.

Prime Minister Samak Sundaravej spoke on Friday after two meetings with the country's military chiefs to discuss the increasing unrest in Bangkok's streets caused by protesters seeking his resignation.

Samak says he had ordered police to withdraw from confronting demonstrators because he hoped to calm down the situation, in which both sides scuffled roughly in the street.

But he says that after he hosts a ceremony honouring Thailand's royal family on Saturday, he will consider declaring a state of emergency if the situation deteriorates.

That would allow suspending some legal procedures to restore the peace.

Meanwhile, Thai police fired tear gas on Friday to try to break up about 2,000 anti-government protesters gathered outside the Bangkok police headquarters, an Agence France-Presse correspondent and witness said.

Thai television also carried reports that tear gas was fired, and said the protesters quickly dispersed afterwards.

Sondhi Limthongkul, one of the protest leaders, had earlier called on thousands of people currently besieging Bangkok's main government compound to march to the nearby Bangkok Metropolitan Police headquarters.

The crowd was demanding that police turn over any officers responsible for brief clashes at Government House earlier Friday, which caused minor injuries to a handful of protesters, witnesses said.

"We want the police to send out the policemen who gave the orders and the police who hurt people," Sondhi earlier said.

"We ask them to come out by 7pm (2200 AEST) otherwise we will bring 100,000 protesters to surround the police headquarters."

Police officials were not immediately available to confirm the use of tear gas, but earlier in the day riot police clashed with protesters at the government compound, using shields and batons.

Supporters of the People's Alliance for Democracy have been barricaded in the extensive grounds of Government House since Tuesday, demanding Samak's resignation.

Up to 25,000 demonstrators have barricaded themselves in the government complex, accusing Samak of being a figurehead for ousted premier Thaksin Shinawatra and insisting he must step down.

The anti-government campaign also appeared to be broadening, with protests forcing the temporary closure of three airports in popular tourist destinations in southern Thailand.