Hundreds of riot police have entered the compound of the Thai prime minister's office to deliver a court order for the eviction of protesters trying to overthrow the government.
The police posted the order on lampposts and barricades in the compound and surrounding streets in Bangkok.
Scuffles broke out and witnesses said teargas was fired, but police deny this.
Thailand's prime minister said on Thursday he would not use violent means to drive out protesters occupying a government zone, in an apparent change of heart.
[Protest leaders have vowed to stay until Samak Sundaravej's administration falls.
Despite his record as an instigator of a bloody crackdown on left-wing students in 1976, Mr Samak has blown hot and cold against the protest, threatening variously the "full force of the police", a siege, or nothing at all.
The protesters accuse Mr Samak of being a puppet of former Prime Minister Thaksin Shinawatra, ousted by a military coup in 2006.
Mr Samak has repeatedly changed his approach to the protesters - prevaricating between advocating a "softly, softly" approach and vowing to take a harder line.
The protests began on Tuesday, when more than 30,000 people took to the streets in a well-co-ordinated action.
They have been called by a group calling itself the People's Alliance for Democracy - formed three years ago as a movement to bring down Mr Thaksin.
Mr Samak has vowed not to bow to the PAD demands for his resignation.
