By Sunday evening the rally at Sanam Luang field near the royal palace had swollen to 50,000 according to a Special Branch Police official, who said the numbers appeared to have levelled out.
Many carried banners proclaiming "Thaksin get out" and depicting him as Adolf Hitler, the Nazi leader.
They cheered wildly as anti-Thaksin leaders Chamlong Srimuang, a former mentor of the prime minister, and Sondhi Limthongkul, Thaksin's fiercest public critic, took the main stage, which was festooned with a caricature of a monstrous Thaksin devouring the Thai flag.
Chamlong told the protesters: "There are many of us today ... if we unite and be patient I think we will win."
He said: "Thaksin repeatedly said he would not dissolve the house, now he dissolved it, and he repeatedly said that he would not resign. I wonder how can he resist if 100,000 of us protest."
Thaksin's decision on Friday to dissolve the parliament and hold elections in April, three years earlier, failed to appease critics who have vowed to continue pushing him to resign over corruption allegations.
Sondhi accused Thaksin of calling the snap polls to deter the public from rallying against him, saying "he is still talking brave, but his legs are shaking".
The three main opposition parties are now seeking a meeting with Thaksin to persuade him to commit in writing to constitutional reforms in an attempt to end the deepening political crisis.
"The current political tension is very serious... We have not yet ruled out a boycott of the election"
Leader, Democrat party
Abhisit Vejjajiva, leader of the opposition Democrat party who did not attend Sunday's rally, said: "The best solution as of now is to hold a meeting of the four party leaders, including (Thaksin's) Thai Rak Thai party, on Monday to consult on constitutional changes to go ahead with political reform."
If Thaksin agrees to review the constitution, the opposition is proposing the installation of a neutral interim prime minister named by the king.
But the opposition has not ruled out a boycott of the 2 April election.
Abhisit said: "The current political tension is very serious ... We have not yet ruled out a boycott of the election."
Thaksin said he would send two deputies to meet the opposition, without saying whether he would agree with their constitutional reform proposal.
He said: "Whatever makes the country peaceful and reconciled is good. I myself never obstruct political reform if it is good."
Sondhi Limthongkul is Thaksin's
fiercest public critic
The anti-Thaksin movement gained momentum over his family's $1.9 billion tax-free sale of stocks in Shin Corp - the telecoms giant he founded before entering politics - to foreign investors in January.
The Alliance mobilised 50,000 people for a 4-5 February rally in Bangkok in the biggest anti-government protest since Thaksin took office in 2001. A week later, 20,000 people gathered in the capital.
About 4700 unarmed police officers were mobilised for Sunday's demonstration, officials said.
But no incidents were reported, as sporadic chants of "Get Out" erupted from the noisy but peaceful crowd.
Tatawan Nimpijarn, a 30-year-old businesswoman, said: "I want Thaksin to get out."
Tatawan said: "He changed many laws just for his own benefit."
The only disruption came when a generator caught fire near the main stage, briefly stopping the speakers.