Two exit polls were released after polls closed at 3 p.m.
One, the Suan Dusit University poll, gave Pheu Thai, Ms. Yingluck’s party, 313 of 500 parliament seats, compared with 152 seats for Mr. Abhisit’s Democrat party. Another poll by Bangkok’s Assumption University gave Pheu Thai 299 seats compared to 132 for the Democrats.
Pheu Thai needed more than 250 seats to form a government without help from smaller parties.
The photogenic Ms. Yingluck long has been seen as the front-runner in the vote. Her popularity is almost entirely due to fact that she is the proxy of Mr. Thaksin, who has been legally barred from politics after his conviction on corruption.
His ascent to power in 2001 changed Thailand forever, touching off a societal schism between the country’s haves and long-silent have-nots. The marginalized rural poor hail his populism, while the elite establishment sees him as a corrupt, autocratic threat to the status quo and even to the revered constitutional monarchy.
That schism has played out through pro- and anti-Thaksin street protests since the 2006 coup. The vote, many believe, is largely about the divisive legacy he left behind.
For a nation of 66 million people known to tourists as “the Land of Smiles,” much is at stake.
Last year’s demonstrations marked some of the nation’s worst violence in two decades and left Thailand’s reputation for stability in tatters. Holding the ballot was one of the protesters demands, though they wanted it held last year.
Oxford-educated Mr. Abhisit used his campaign to blame the opposition and its supporters for burning Bangkok last year, saying a vote for Ms. Yingluck would be a vote for chaos. He also declared the poll “the best opportunity to remove the poison of Thaksin from Thailand.”
Mr. Abhisit and his allies believe Ms. Yingluck is plotting Mr. Thaksin’s return through a proposed amnesty that would apply for political crimes committed since the coup. Ms. Yingluck says it is aimed at reconciling all Thais, not just her brother.
Mr. Thaksin has vowed to return by year’s end, but he said Sunday that “I have to be part of the solution … I don’t want to return and create problems. If that’s the case, I don’t have to go back yet.”
Sunai Phasuk, a senior researcher for Human Rights Watch, said the most important challenge facing the incoming government will be resolving the nation’s divide.
“Everyone is talking about political deals, but no one is talking about how to end impunity, restore freedom of expression and hold perpetrators accountable no matter how high up they are,” Mr. Sunai said. “Without that, Thailand will never able to get out of this cycle of violence and turn itself around.”
Although Mr. Thaksin is credited for awakening what has become a democratic movement among the country’s marginalized poor who long stood silent, his opponents say he is no champion of freedom. During his time in office, Mr. Thaksin was loudly criticized for a sharp authoritarian streak and stood accused of corruption, cronyism and abuse of power.
Associated Press writers Grant Peck and Sinfah Tunsarawuth contributed to this report.