Former president Mahinda Rajapaksa's attempt to stage a comeback in Sri Lanka's general election has ended in defeat as results on Tuesday showed the alliance that toppled him making decisive gains.

The ruling United National Party (UNP) was likely to fall just short of an outright majority but Prime Minister Ranil Wickremesinghe should still command enough support to form a stable government.

"I invite all of you to join hands," Wickremesinghe, 66, said in a statement as the count neared completion. "Let us together build a civilised society, build a consensual government and create a new country."

If confirmed, the outcome would be a triumph for President Maithripala Sirisena, who beat his former ally Rajapaksa in a presidential vote in January and called early parliamentary polls to secure a stronger mandate for reforms.

Defeat for Rajapaksa will keep Sri Lanka on a non-aligned foreign policy course and loosen its ties with China, which during his rule pumped billions of dollars into turning the Indian Ocean island into a maritime outpost.

With results from 18 of Sri Lanka's 22 districts in, Wickremesinghe's UNP had won 70 seats, one more than the tally of the election alliance led by Rajapaksa's Sri Lanka Freedom Party (SLFP).

UNP sources and political analysts said the lead was likely to widen, with the party on track to win 105-107 seats, just shy of a 113-seat majority.

A total of 196 seats are up for grabs in multi-member constituencies with a further 29 to be allocated by proportional representation in the 225-seat chamber.

FAILED COMEBACK

Nationalist strongman Rajapaksa set his sights on becoming premier of an SLFP-led government but Sirisena, who succeeded him as party leader in January, has ruled that out and purged Rajapaksa loyalists from senior posts.

A group of Sirisena followers is expected to cross the floor to join a broad-based national unity government led by Wickremesinghe, who was likely to be confirmed in his post on Tuesday.

"The UNP will not have an overall majority - it will have to look for coalition partners from those who support Sirisena," said Paikiasothy Saravanamuttu, head of the Centre for Policy Alternatives.

With outside support, the centre-right alliance could hope to muster the two-thirds majority required to pass proposed constitutional reforms that would make government more accountable and simplify Sri Lanka's complex election laws.

The power struggle between the past and present presidents overshadowed the election in a country with a history of political feuding that has often spilled over into violence and even the assassination of its leaders.

A broad backlash against Rajapaksa's attempt to win an unprecedented third term led support to coalesce around his former health minister Sirisena, a humble figure with none of the muscular bravado of his predecessor.

The 69-year-old Rajapaksa, expected to lead a rump parliamentary opposition, told Reuters earlier he would "support good policies and oppose bad things".

Yet he could now be confronted with a judicial reckoning, along with two brothers who held high office, for alleged corruption and abuse of power during his decade in power. They have denied any wrongdoing.

"Mahinda has to compromise - resign from politics and parliament, and settle down as a former president - or face the legal consequences," said a Sirisena aide, who spoke to Reuters on condition of anonymity.  - Reuters