Rajapaksa is confident of returning to power after drawing massive crowds on the election trail, despite the 69-year-old and his closest relatives facing corruption allegations.
"This election is about whether you want Mahinda Rajapakse as prime minister or not," his spokesman, Rohan Valivita, said.
"He is 100 percent confident of victory" at the poll for the national Parliament.
His successor as president, Maithripala Sirisena, has vowed to thwart his one-time mentor's ambitions to become prime minister.
But even Rajapaksa's opponents agree the election has turned into a referendum on the charismatic leader who ruled Sri Lanka for nearly a decade until his surprise defeat at a presidential poll in January.
The two men had been allies in their ruling United People's Freedom Alliance (UPFA) until late last year when Sirisena quit as health minister to run for the presidency.
Polling booths opened across the island at 7:00 am (0130 GMT), with voting set to continue for nine hours under tight security provided by 74,000 police and paramilitary forces, officials said.
Hundreds of voters had lined up outside some of the 12,300 polling booths across the country before they opened, officials said adding strict security was being enforced to ensure a free and fair election.
Around 15 million men and women over the age of 18 years are eligible to vote in the election for a 225-member parliament.
Although there are no reliable opinion polls, no single party is expected to win a majority.
Observers say Rajapaksa's polarising personality will undermine his chances of forming a coalition, especially as any potential prime minister would likely need the backing of minority groups.
Rajapaksa is hugely popular among big sections of the ethnic majority Sinhalese community for presiding over the crushing defeat of Tamil guerrillas in 2009 after their 37-year campaign for a separate homeland.
But he is also reviled by many minority Tamils who voted en masse for Sirisena in January after boycotting previous elections.
Although Sirisena is now UPFA leader, his reluctant agreement to Rajapaksa's candidacy highlighted his shaky hold on the party.
Sirisena is thought to prefer outgoing premier Ranil Wickremesinghe's United National Party (UNP) to form the next government with backing from Tamil and Muslim parties.
"People don't want Rajapaksa to come back," Wickremesinghe said at his final press conference before the vote. Rajapaksa's return is "an attempt to resurrect the politically dead," he said.
Since his defeat in January, Rajapaksa has seen his wife and two of his brothers accused of corruption, while one of his sons has been implicated in the alleged murder of a former rugby star.
Sirisena last week dashed Rajapaksa's hopes of a comeback by vowing not to appoint him as premier even if their party won -- and will instead choose one of seven others running for the post.
"If Rajapaksa does very well at the election, Sirisena will have no choice but to make Rajapaksa his prime minister," political analyst Kusal Perera said. "However, I don't think the question will arise."
"I think Rajapaksa's comeback effort will leave him as the leader of the opposition at the most."
The results are expected to be released Tuesday.
Rajapaksa was shunned by Western governments over the brutal end to the island's ethnic conflict which prompted calls for international investigators to carry out a war crimes probe.
The UN says that some 40,000 Tamil civilians were killed in the final stages of the war, one of the bloodiest in Asia in the post-colonial era.
Rajapaksa's campaign speeches have been peppered with claims that Sirisena is "selling out" to Tamils and Muslims who together account for a quarter of the population.
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