Nearly 47.3 million Thais have voted in a general election that could prove pivotal to the political future and stability of the Southeast Asian country.
Sunday's poll is the first major electoral test for the government since mass opposition rallies in Bangkok, the Thai capital, last year, which sparked a military crackdown that left around 90 people dead.
Exit polls showed the main opposition Pheu Thai party heading for a landslide victory against the governing Democrat party.
Pheu Thai is expected to win 299 of 500 seats, according to an exit poll by the Assumption University of Thailand.
Another figure by Nida Poll gave 275 seats to Pheu Thai and 104 to the Democrats.
Yingluck Shinawatra, a sister of former prime minister Thaksin Shinawatra, heads Pheu Thai. Abhisit Vejjajiva, the incumbent prime minister, is the leader of the Democrat party.
Red Shirts movement
Thaksin has lived in self-imposed exile in Dubai since being deposed by the military on charges of corruption in a 2006 coup.
His "Red Shirt" supporters rallied in central Bangkok last year, bringing the area to a standstill for two months.
The capital's Rajaprasong intersection was the focal point of those demonstrations. Demonstrators set buildings on fire, forced businesses to shut down and clashed with security forces.
The Democrat party wants Shinawatra to return to the country to stand trial for corruption.
Earlier on Sunday, Aela Callan, reporting for Al Jazeera from Khon Kaen, in northern Thailand, said: "There are 90,000 polling stations ... across Thailand open now, and 180,000 police have been deployed in order to keep the peace. We're not expecting any widespread violence, certainly not on the scale of what we saw in Bangkok last year."
Vejjajiva took office in 2008 after a parliamentary vote, following a court ruling that threw out the previous administration. His party has not won a general election in nearly two decades.
Both major parties have presented similar populist campaigns during the six weeks leading up to the polls, focusing on subsidies for the poor, improved healthcare and investment in infrastructure.
The election is Thailand's 26th since it became a democracy in 1932, before which it was an absolute monarchy.
The country has been governed under 17 constitutions and has had 18 military coups [both actual and attempted].
Aela Callan reports from the Red Shirt stronghold of Khon Kaen in northern Thailand in the run-up to Sunday's general election
Our correspondent reported that on Sunday morning, there had been three arrests over allegations of vote buying, and that election observers from both parties were out in force.
The fear of violence centred on whether the Red Shirts perceive that either vote rigging cost them seats, or if they are not allowed to form a government.
"If they cheat there will be protests in Bangkok for sure. It will be the same as the Rajaprasong protest," said Tan Chaithep, chief assistant of Ban Nong Hoo Ling, a village in Thailand's northeast known to be a "red shirt" stronghold.
Thailand has about 47 million eligible voters, who will select 500 members of the lower house of parliament (the House of Representatives).
Polling opened at 8am local time (01:00 GMT) and closed at 3pm local time (08:00 GMT).
Phra Rakkiart, a former public health minister, has warned that the polls will go down as the "dirtiest in history".
"Canvassers sell lottery tickets to voters and offer them a big reward if their candidates win in the election," the Bangkok Post quoted him as saying, adding that election fraud was ingrained.
Suranand Vejjajjiva, a cousin of Abhisit who formerly served in Thaksin's cabinet, cast the elections in a more hopeful light, saying that Sunday morning's high voter turnout is an indication of the Thai people's desire for democracy.
"In the past two decades, although yes, we had a coup in 2006, I think we have learned a lesson, the Thai society has learned a lesson," Suranand told Al Jazeera.
"So any interference, any non-acceptance of the elections will not be accepted by the Thai public."
He said that while he personally wishes for Thaksin's return, a Pheu Thai victory will not guarantee that.
"What everyone wants is [that] the new government ... can assure not just Thaksin but everyone whose top political activists were alleged to have done something wrong, could depend on a reliable and just judicial process," Suranand said.
"If Thaksin wants to come back then he has to argue his case against court, then the court will have to determine whether he can come back or not," Suranand said.
"But I'm sure it's not a simple case that Pheu Thai wins and Thaksin can come back right away."