The red-shirted followers of ousted Prime Minister Thaksin Shinawatra say Prime Minister Abhisit Vejjajiva's government came to power in December illegally and are demanding Parliament be dissolved and new elections held.
The protest began after Abhisit began a "help-the-nation" program in which the government gave $57 checks to more than 9 million people earning less than $424 a month as part of a $509 million economic stimulus package -- a move critics denounced as a vote-buying payoff.
Thaksin, a billionaire ousted in a 2006 coup and living in exile to avoid a two-year jail sentence for corruption, thanked the crowd by telephone and urged supporters to topple Abhisit's government.
"I will tell you who is behind the regression of Thai politics for the past 30 years tomorrow," Thaksin said. "And I will talk about the problems with the economy on Saturday."
Protester Jatuporn Prompan, an opposition parliament member and a leader of the pro-Thaksin United Front for Democracy Against Dictatorship, told the Thai News Agency that the rally would go on for at least a month and that the group could sustain protests for a year.
The British-born Abhisit told reporters he had given instructions to the 10,000 deployed police and soldiers to be "extra patient because there are expected provocations."
The demonstrators promised not to enter the compound, CNN reported.