A man walks past a poster of Sri Lanka's former president Mahinda Rajapaksa ahead of a general election, in Galle August 14, 2015. [Photo/Agencies]

COLOMBO - Sri Lankans went to the polls on Monday to elect a new parliament in what amounts to a referendum on ex-president Mahinda Rajapaksa's comeback bid, with the reformist alliance that swept him from power seeking a stronger mandate.

The nationalist strongman has set his sights on becoming premier of a government led by his Sri Lanka Freedom Party (SLFP). But the former ally who beat him at the polls in January, President Maithripala Sirisena, now leads the party and he rules that out.

The tangled personal rivalry has overshadowed campaigning on the Indian Ocean island of 20 million people, which has a history of political feuding that has often spilled over into violence and even the assassination of its leaders.

Sirisena, in a cross-party alliance with a government led by the United National Party (UNP), has sought to break with that troubled past by passing reforms to weaken his own presidency and make the government more open and accountable.

Some voters in Colombo said they were casting their ballots for reconciliation and good governance, showing sympathy for the UNP of Prime Minister Ranil Wickremesinghe.

"I came to vote to have just and fair governance ... for people to live like humans," lawyer Rushdi Halid told Reuters.

Minority Tamils and Muslims have rallied behind the UNP-led alliance, which pundits say has the best chance of forming the largest bloc in the 225-seat parliament and advancing reforms that have stalled because it lacks a majority.

Wickremesinghe said he was very confident of beating Rajapaksa. "He has lost already," he told reporters after voting. "I haven't got to worry any more about Mahinda Rajapaksa - in a free and fair election we can hold him."