COLOMBO (AFP) - Sri Lanka's main opposition candidate took an early lead over incumbent Mahinda Rajapakse in the island's bitterly contested presidential election, partial results showed on Friday.

Mr Maithripala Sirisena was leading with about 56 per cent of the nearly 600,000 votes counted so far, with Rajapakse trailing at 41 per cent, according to the Department of Elections.

Just over 75 per cent of the 15.04 million electorate turned out to vote in Thursday's election called two years ahead of schedule.

Mr Rajapakse had seemed assured of victory when he declared the snap poll in November seeking an unprecedented third term, five years after crushing a violent separatist rebellion that 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.

Turnout was higher than expected in Tamil-dominated areas of the north and east, which have boycotted national elections in the past and where resentment against Rajapakse runs high.

Tamils - Sri Lanka's largest minority, accounting for 13 per cent of the population - appeared to have voted overwhelmingly in support of Sirisena.

Mr Sirisena was a relative unknown until he became the main opposition candidate, but his decision to run triggered a slew of defections and become a rallying point for disaffection with Rajapakse and his powerful family.

Election commissioner Mahinda Deshapriya said final results were expected later on Friday.