The "red shirt" protesters, expected by city officials to exceed 80,000, are calling for new elections and the resignation of Prime Minister Abhisit Vejjajiva, The New York Times reported. Abhisit said he would not resign.
He said security forces would avoid confrontations with the protesters but warned "the government will have to do something" if violence erupts.
Thousands of protesters camped outside Government House, blocking access to government offices for two weeks, the Times said.
The demonstrators are supporters of former Prime Minister Thaksin Shinawatra, who was elected in 2001 and re-elected in 2005 but ousted in a military coup in 2006. Thai newspaper The Nation reported Wednesday several thousand people surrounded the home of Gen. Prem Tinsulanonda, whom Thaksin accused of orchestrating the coup.
Thaksin, a former telecommunications billionaire, fled the country before he was convicted last year on abuse-of-power charges and lives in exile, mainly in Dubai. Thaksin faces other charges in Thailand and the courts have frozen an estimated $2 billion in family assets.
Speaking via video hookup, Thaksin urged supporters to "gather in large numbers and show your power."
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