Thailand's embattled prime minister will step down after opposition forces said they would renew protests a day after his party claimed victory in weekend elections.

"I am sorry that I will not accept the premier post," Thaksin Shinawatra told a news conference Tuesday.

On Monday, Shinawatra's Thai Rak Thai party claimed victory after winning 57 per cent of Sunday's ballots. The support was down from elections last year, but Thaksin said at the time he saw no reason to resign.

Thaksin Shinawatra had been under pressure for months to resign his post as prime minister.(Associated Press file)
Thaksin Shinawatra had been under pressure for months to resign his post as prime minister.(Associated Press file)

However, opposition groups say the vote was tainted by the fact that 10 million people checked "no vote" on their ballots, including a majority in the capital Bangkok. Opposition parties also boycotted the vote, refusing to run candidates in 278 constituencies.

Thaksin's critics have been staging rallies for the past two months, drawing as many as 100,000 people. They've been calling for him to step down or hold new elections and rejected his proposition for a reconciliation committee to try to solve the political crisis.

Protests erupted in Bangkok in January after it was revealed that the billionaire prime minister had avoided paying taxes when he sold a stake in his family's telecom firm.

Thaksin paid a visit to King Bhumibol Adulyadej prior to making Tuesday's announcement.

Thaksin said he would remain in a caretaker role until a successor is chosen, which could happen shortly after parliament resumes next month.