Buddhist monks of the "Dharma Army" were in the crowd and there were shouts of "Thaksin Get Out" as a huge rally got under way on Sunday in the Thai capital's Sanam Laung square near the Grand Palace.
Spearheaded by the People's Alliance for Democracy - a coalition of anti-Thaksin groups - demonstrators are rallying to force the prime minister out of office. The alliance accuses Thaksin of gutting democratic institutions, cronyism, tax evasion and serious human rights violations
Suriyasai Katasila, an alliance leader, said: "The rally will go on and we won't stop unless we win."
Organisers hoped to draw tens of thousands despite the Thai leader's refusal to bow to "mob rule".
Lakhana Promchokewattana, 27, a businesswoman from the central province of Ratchaburi, said:
"The prime minister is corrupt. If this prime minister is as good as he claims, why are so many people from all walks of life sitting here and braving the heat to protest against him."
"If this prime minister is as good as he claims, why are so many people from all walks of life sitting here and braving the heat to protest against him"
Lakhana Promchokewattana, Bangkok protester
The rally was joined by Thaksin's former political mentor, retired general Chamlong Srimuang, who said he would lead his Dharma Army later to the nearby Democracy Monument and camp there until dawn.
"We have not decided whether we will move anywhere else," said Chamlong, who led a successful but bloody "people's power" revolt against a military-led government in 1992.
Thaksin Shinawatra rejects calls
for his resignation
Police searched handbags for weapons at checkpoints around the square. A 100-strong anti-riot squad, armed with batons and shields, kept a discreet distance.
Police Colonel Pinij Maneerat said: "Police are under instructions not to confront protesters or stop them, even if they march to the prime minister's home or Government House."
Opposition parties have said they will boycott forthcoming elections, that Thaksin - whose populist policies have earned him widespread popularity in rural areas - will almost certainly win.
"Thaksin's proposal for political reform is mere lip service. We cannot trust him any more"
Abhisit Vejjajiva, opposition politician
Thaksin was re-elected to a second term last year when his Thai Rak Thai party won 377 of the 500 seats in the House of Representatives. His main defence against critics is that he enjoys a mandate endorsed by 19 million voters.
More than 100 academics will, meanwhile, submit a petition to King Bhumibol Adulyadej on Sunday, seeking a royally appointed prime minister who would run the country until general elections can be properly staged.
Thaksin has already begun his election campaign and at a rally on Friday of more than 100,000 supporters he offered to resign if he failed to secure more than half the votes in the polls next month.
If he is re-elected, he said, he will hold a national referendum on constitutional reforms within 15 months and then call fresh elections. He also said that instead of boycotting the poll, voters should tick the "no vote" on their ballot to reflect their decision to abstain.
"I play by rules someone else wrote, so come join the election with me," he said, maintaining that he was following democratic procedures.
But the leading opposition parties say they will not take part. Abhisit Vejjajiva, the Democrat party leader, said: "Thaksin's proposal for political reform is mere lip service. We cannot trust him any more."
Tens of thousands of protesters have been demanding Thaksin's resignation in regular weekend rallies, accusing him of corruption, mishandling a Muslim insurgency in southern Thailand, stifling the media, and allowing cronies to reap gains from state policies.