Sirisena has vowed not to make Rajapaksa premier even if his United People's Freedom Alliance (UPFA) wins a majority.
Polls opened at 07:00 local time and will close at 16:00.
"Island wide 12,314 polling stations and 1,600 counting centres have been set up for today's election. Around 195,000 officers are involved in election duty. Out of them 125,000 officials are deployed at polling stations, while another 70,000 officials are deployed at the counting centers," Deputy Elections Commissioner M M Mohammed said.
The election to the 225-seat national parliament promises a close battle between the United National Party (UNP) of Prime Minister Ranil Wickremesinghe and the UPFA of President Sirisena.
Over 15 million voters are eligible to vote in the election being held under electoral districts-based proportional representation system.
196 members will be elected from districts while 29 will be appointed based on the national proportion of votes polled by each party.
To have a working majority government, 113 seats are needed in the National Assembly.
The rivalry between Wickremesinghe and Sirisena is limited to party positions they hold as the former's support helped the latter become president in January by defeating Rajapaksa.
Rajapaksa, 69, forced himself into the UPFA reckoning to contest the parliamentary poll, an action unprecedented for a former president of the country.
Sirisena was not in favour of granting Rajapaksa a party ticket but his party allies have defied his wishes.
Accusing Rajapaksa of alienating Tamil and Muslim minorities from the Sri Lanka Freedom Party (SLFP), Sirisena had last week asked his predecessor not to create divisions in the party.
Sirisena was Rajapaksa's health minister until he came forward as the opposition unity candidate to challenge the then president last year. He then handed Rajapaksa a shock defeat in the polls.
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