Prime Minister Thaksin Shinawatra called the election three years early to reassert his mandate after weeks of persistent street protests against his government. But the vote was unlikely to satisfy critics who accuse him of corruption and abuse of power.All three opposition parties in Parliament boycotted the election, urging Thailand's 45 million registered voters to cast ballots but tick a box indicating they abstain from choosing a candidate.

Thaksin has promised to step down if his party Thai Rak Thai party receives less than 50 percent of the vote, an outcome seen as highly unlikely given his strong support among the rural majority. The opposition, however, hopes the boycott will make it impossible to fill all 500 legislative seats, which many legal experts say could make it impossible to convene Parliament and form a new government.

Looking relaxed and smiling, Thaksin cast his ballot at a voting station near his residence in western Bangkok, accompanied by his three children.

"This election is very important for the direction of the country because there is a split right now," he told reporters. "Voting is the responsibility of everyone who loves democracy. They must come out and vote."

The opposition boycott turned the election into a one-party race in 276 constituencies where Thaksin's party was running uncontested.

The ruling party won an overwhelming mandate in elections last year and remains popular among rural Thais who have benefited from Thaksin's social welfare programs.

But election law requires that uncontested candidates garner at least 20 percent of votes highly unlikely in some districts where Thaksin's party has little support. Any seats left unfilled could lead to several rounds of by-elections before a prime minister is chosen.

Abhisit Vejjajiva, head of the main opposition Democrat Party, said the boycott sent a strong message.

"What we did when we took the position was to make a statement, and we made a statement," he said, calling for a close examination of the vote to rule out fraud. "We have to make sure the elections are fair no fraud, no irregularities."

Chaiyan Chaiporn, a prominent political science professor at Chulalongkorn University, made a show of ripping up his ballot card at a central Bangkok polling station, as other voters erupted in applause. He was promptly arrested but released on bail. The act is punishable by one year in prison, a $500 fine and a five-year revocation of voting rights.

The anti-Thaksin movement adopted the slogan "Vote for No Vote," and urged people to wear black on election day as a protest symbol.

Protesters say they will keep up the battle to force him out of office regardless of the results of the poll.

The movement against Thaksin gained mass support in January when the prime minister's family announced it had sold its controlling stake in telecommunications company Shin Corp. to Singapore's state-owned Temasek Holdings for a tax-free $1.9 billion.

Critics allege the sale involved insider trading and complain a key national asset is now in a foreign government's hands. He is also accused of mishandling a Muslim insurgency in southern Thailand, stifling the media and allowing cronies to reap gains from state policies.