Rajapakse called the election two months ahead of schedule after his resounding victory in a presidential vote in January.
Opposition parties have accused Rajapakse's United People's Freedom Alliance (UPFA) of violating campaign norms and warned that Thursday's vote would not be free and fair, with his main political rival also behind bars.
But the charges are unlikely to have any major impact on voter intentions, which show a clear preference for the ruling party as it basks in the glow of Rajapakse's victory over the Tigers last May.
The crushing of the Liberation Tigers of Tamil Eelam (LTTE), which had been fighting for an ethnic Tamil homeland since 1972, was a major factor behind Rajapakse's re-election in January.
"We defeated the conspiracy to divide the country," Rajapakse told a rally Monday. "Now is the time to make Sri Lanka the wonder of Asia."
While the UPFA is predicted to win more than half the 225 seats in parliament, it is unlikely to secure the two-thirds majority Rajapakse has called for in order to amend the constitution.Despite the Tigers' defeat in May, security will be tight, with 20,000 troops on duty to reinforce police at polling stations around the country.
More than 14 million people are eligible to vote, with a total of 7,620 candidates from 36 political parties and 10 independent groups running for parliament.
Rajapakse's nationalistic rhetoric appeals to the majority Sinhalese community to which he belongs, but has been criticised by rights groups who accuse the president of suppressing dissent as he seeks to expand his already substantial power.
Less than two weeks after his re-election, Rajapakse ordered the arrest and detention of his defeated rival, former army chief Sarath Fonseka. Dozens of Fonseka supporters were also taken into custody.
Fonseka, who led Rajapakse's military campaign against the Tamil Tigers, is currently under court martial and will contest the vote from his cell at Navy headquarters.
Opposition parties were largely united behind Fonseka in his campaign for the presidency, but his arrest broke the fragile bond keeping them together, hampering their ability to provide a real challenge in the parliamentary poll.
On the eve of voting, the main opposition United National Party (UNP) accused the government of using state-owned vehicles and buildings for campaigning and turning the government-run media into a party mouthpiece.
"There was suppression of private media. Journalists were attacked and abducted by those connected to the government," UNP leader and former prime minister Ranil Wickremesinghe told reporters.
"Some feel there is no point in voting because their votes will not make a difference due to rigging," Wickremesinghe said. "We appeal to the people to go and vote. Please. There is an opportunity to change the government."
He accused the police of failing to implement election laws and favouring ruling party candidates, a charge already denied by the authorities.
Elections Commissioner Dayananda Dissanayake has gone on record saying the police and civil servants ignored some of his directives to ensure a fair poll.
Wide expectations of a ruling party win has fuelled predictions of a low turnout following a campaign that the private Sunday Island weekly called the "most excruciatingly boring" since independence.