Protesters converged Wednesday on Government House, the headquarters of Prime Minister Samak Sundaravej, for what they called a final push to remove him from office.
"I'm ready to be arrested," protest leader Chamlong Srimuang told The New York Times. "If they want to put us in jail, we'll go to jail. We aren't going anywhere."
The BBC said riot police had moved through the protesters to take up positions. There were some minor scuffles during the deployment, but the situation remained peaceful overall.
The upheaval is described as fallout from the 2006 coup in Thailand that ousted Prime Minister Thaksin Shinawatra. The Times said the incumbent Samak is viewed as a puppet of Thaksin by the demonstrators.
The Times also noted Thailand's English-language newspapers have criticized the protest as having no clear agenda or plan for the nation should Samak be pushed out of office.