Tens of thousands of protesters rallied in Bangkok in their biggest bid yet to topple Premier Abhisit Vejjajiva, sparking fears of a violent new twist to Thailand's political crisis.

Security forces guarded key government locations as supporters of fugitive former prime minister Thaksin Shinawatra massed here, one day after activists raised tensions by attacking Abhisit's car and smashing a window.

Police said around 60 000 red-clad protesters chanting "Bring Thaksin back, Abhisit get out!" gathered outside Abhisit's office in the capital, where demonstrators have been staging a sit-in for the past two weeks.

A potential flashpoint arose when they later surrounded the house of King Bhumibol Adulyadej's top adviser, General Prem Tinsulanonda, who has been accused by Thaksin of orchestrating the coup that toppled him in 2006.

"We came here to expel the government," protest leader Nattawut Saikuar told the cheering crowd. He added that Prem should also quit for "inciting the military to turn against democracy."

Dozens of soldiers stood guard inside the adviser's house.

The accusations against Prem have broken a major taboo in Thai society, where the royalty is revered, and fuelled the risk of what Abhisit described earlier this week as a "civil war" after more than three years of instability.

British-born Abhisit warned the protesters of strong action if there was any violence, and rejected their demands to dissolve his four-month-old government and hold fresh elections.

"The government will act decisively with any provocateurs," he said. "I will not dissolve the house because of violence."

Abhisit said that some protesters wanted to trigger "chaos on the streets", adding that the attack on his motorcade showed that there were deliberate efforts to provoke the government.

The unrest has prompted mounting speculation that the army could try to mount another coup, despite the fact that Abhisit and his ministers are accused by their opponents of being the military's stooges.

But powerful army chief General Anupong Paojinda ruled out talk of a putsch. "No matter how the situation deteriorates, we will abide by the law and use no other extra power," he said.

The so-called "Red Shirts" remain furious about the way Abhisit took power in December, after a court decision that removed billionaire Thaksin's allies from government.

That ruling came after months of protests by rival, yellow-clad protesters claiming allegiance to the monarchy, who occupied Government House and mounted a crippling blockade of Bangkok's airports late last year.

Thaksin, who is living in an undisclosed foreign country to avoid a prison term for corruption, promised that the protests would mark a "historic day for Thailand."

"We will come peacefully but we need as many people as possible to show that the Thai people will not tolerate these politics any more," he said in a speech via videolink to supporters outside Government House late Tuesday.

Major General Suporn Phansua, a spokesman for Bangkok Metropolitan Police, said around 10 000 security forces had been deployed at major sites including Government House and Prem's residence.

The nation remains deeply divided between Thaksin's followers, mainly among the urban and rural poor in northern Thailand, and his foes in Bangkok's traditional power cliques of the palace, military and bureaucracy.

The government is under extra pressure to keep the peace ahead of a key summit of Asian leaders due to start Friday in the resort town of Pattaya ? the place where Abhisit came under attack Tuesday.