Critics of Thai rallied for the fourth time in a month in Bangkok to demand constitutional reform.
The crowd Sunday chanted for the prime minister to get out. Their leaders said they would maintain a vigil until he does, and would march on his offices and home to press their demands.
Hundreds of police were on alert late Sunday, amid fears of violence, and security was tightened outside Government House and the prime minister's residence.
The prime minister, responding to charges of corruption and abuse of office, 10 days ago, dissolved parliament and called snap elections for April second.
Mr. Thaksin Friday kicked off his campaign with his own rally in Bangkok.
The estimated 100,000 people chanted for Mr. Thaksin to fight his critics.
Mr. Thaksin told them if he does not receive at least one-half of the votes in next month's election, he will step down.
He says, if elected, he will launch a political reform process, which should take nine-to-15 months. Then he will call fresh elections.
The three main opposition parties, however, vow to boycott the April election, saying Mr. Thaksin's party has too much power for the election to be fair. They want a neutral government to be appointed to reform the constitution, and then, oversee elections.
Thailand's urban middle class has become increasingly critical of Mr. Thaksin, one year after he was re-elected by a landslide.
The criticism intensified six weeks ago, after his family sold nearly $2 billion worth of stock in the company he founded, without paying taxes. The tax-free sale was legal, but angered many Thais.
Mr. Thaksin continues, however, to enjoy widespread support among rural people and the poor, who have benefited from his populist policies.
The opposition has focused on the issue of revising Thailand's constitution, which was enacted nine years ago after bloody demonstrations in 1992 against the military government.
A professor at Bangkok's Chulalongkorn University, Thitinan Pongsuthirak, says the problem is not the constitution.
"The constitution itself is a solid constitution, but I think the people that have implemented and enforced the constitution have been weak," Thitinan said.
He and other Thai analysts say measures are needed to strengthen regulatory bodies, like the election commission and constitutional court. And they fear that, if the confrontation between the prime minister and his critics continues, it could lead to violence, which both sides say they want to avoid.