Anti-Government protesters crossed heavily fortified barriers to reach Thai Prime Minister Yingluck Shinawatra's office and the Bangkok police headquarters without resistance from police.

Officers even used cranes to remove concrete slabs and barbed wire barricades on a road leading to their headquarters after agreeing to let the protesters into the building.

After bitterly resisting them with tear gas and rubber bullets since Saturday, police just stood by as protesters removed the barriers to the prime minister's office and walked through.

The unexpected reversal of strategy by the government suggests it is alarmed by the reaction to a police clampdown that has led to three days of clashes, three deaths and injuries to more than 230 people.

After breaching the barriers on the road, the protesters milled outside the gates of the prime minister's office in Government House and made no attempt to enter the sprawling compound.

On Monday night, protest leader Suthep Thaugsuban had told his supporters to storm the Bangkok Metropolitan Police Bureau as part of a campaign to topple Ms Shinawatra's government.

At the same time, Ms Shinawatra said that while she was willing to do anything it took to end the violent protests, she could not accept the demand to hand power to an unelected council.

In some of the worst clashes since the protests began last week, demonstrators commandeered rubbish lorries and bulldozers on Monday and tried to ram concrete barriers at Government House and other offices.

Police repelled them by firing tear gas, water cannon and rubber bullets and protesters replied with explosives from home-made rocket launchers.

Instability has plagued Thailand since the military ousted the prime minister's brother and predecessor, Thaksin Shinawatra, in 2006.

Two years later, anti-Thaksin protesters occupied Bangkok's airports for a week and the prime minister's office for three months and in 2010 pro-Thaksin protesters occupied central Bangkok for weeks in a standoff that ended with more than 90 dead.