The Sri Lankan opposition's challenger for the presidency has taken a strong lead over incumbent Mahinda Rajapaksa in early results announced after a partial vote count after Thursday's election.

The Department of Elections said that results from 11 of the country's 160 polling divisions plus a large number of postal votes gave Mithripala Sirisena a lead of 56.5 percent against 42 percent for Rajapaksa, who is seeking a third term as president.

About 600,000 votes have been counted so far. Just more than 75 percent of the 15.04 million electorate turned out to vote.

Rajapaksa had seemed assured of victory when he called snap polls in November seeking an unprecedented third term, five years after crushing a violent separatist rebellion that had traumatised the country for decades.

But he has become unpopular in recent years, dogged by accusations of increasing authoritarianism and corruption and a failure to bring about post-war reconciliation.

His health minister Maithripala Sirisena's surprise decision to defect from the government and stand against him galvanised disparate opposition groups, creating a far closer contest than had been expected.