BANGKOK - Police in Thailand on Monday threatened to disperse supporters of fugitive ex-premier Thaksin Shinawatra, raising tensions as the protesters blockaded the seat of government for a fifth day.

Police warned that it was illegal for the demonstrators to prevent Prime Minister Abhisit Vejjajiva or members of his administration from entering Government House in downtown Bangkok.

Thousands of red-clad protesters have surrounded the sprawling complex since Thursday, fired up by repeated video addresses by the exiled Thaksin, who was ousted in a coup in 2006.

"Police would like leaders and protesters to stop obstructing the entrances once you hear this statement," said the statement issued by the Bangkok metropolitan police.

"If not, police will control the crowd step by step, using only shields in the beginning," it added.

Police estimated around 3,000 protesters were outside Government House early Monday but numbers have swelled to up to 30,000 in the evenings to listen to Thaksin's speeches.

Billionaire Thaksin's allies returned to power in elections the year after he was toppled, leading to a mass protest campaign by his enemies which eventually resulted in Abhisit taking power in December.

Thaksin's supporters remained defiant Monday, calling on the so-called "Red Shirts" to gather in front of city halls nationwide in protest against the police order in Bangkok.

Television footage showed hundreds of Red Shirts massing outside government buildings in several major provincial towns.

"The war between people who love democracy and people who love bureaucracy is almost over," protest leader Jatuporn Prompan told the crowd in front of the prime minister's office.

Thaksin on Saturday urged protesters to "rise up" against the government, while on Friday he dropped a bombshell by accusing two of the revered Thai king's advisers of orchestrating the coup that toppled him.

Abhisit has rejected the accusation.

Thaksin, currently living in exile to avoid a two-year jail sentence for corruption, still attracts widespread support among Thailand's rural poor, while the country's elite accuse him of graft and authoritarianism.

Separately on Monday, 21 leaders of the anti-Thaksin People's Alliance for Democracy (PAD) movement appeared at a police station to hear charges relating to an attempt to blockade parliament last year.

That rally on October 7 ended in bloody clashes between PAD protesters and police in which two people were killed and nearly 500 injured.

Wearing cowboy hats and greeted by hundreds of yellow-shirted supporters, the PAD members faced accusations of illegal assembly, detention and obstructing officials from performing their jobs.

Police must now pass the charges to prosecutors, who will decide whether they will face trial. The PAD leaders left the station after hearing the charges.