Maithripala Sirisena will be sworn in as Sri Lankan president Friday after a shock victory over veteran strongman Mahinda Rajapakse in an election dominated by charges of corruption and growing authoritarianism.

A top aide to Rajapakse said the outgoing president accepted the decision of voters who turned out in force on Thursday, in a remarkable reverse for a leader who had appeared certain of victory when he called snap polls in November.

"The president concedes defeat and will ensure a smooth transition of power, bowing to the wishes of the people," presidential press secretary Vijayananda Herath told AFP.

Sirisena thanked Rajapakse as he formally accepted his victory at the election commissioner's office, saying he had "cleared the way for a fair election that allowed me to be the president".

Commissioner Mahinda Deshapriya declared Sirisena elected with a 51.28-percent share of the vote to Rajapakse's 47.58 percent.

US Secretary of State John Kerry welcomed Rajapakse's early concession and said he looked forward to working with the new leader.

The former health minister, who united a fractured opposition to pull off an unlikely victory, is due to be sworn in on Colombo's Independence Square at 6:00pm (1230 GMT).

Ranil Wickremesinghe, who is set to be Sirisena's prime minister, said all indications pointed to a smooth handover of power.

"People want a new political culture. I don't want anyone taking the law into their own hands," said the opposition United National Party (UNP) leader.

Thilanga Sumathipala, a lawmaker with Rajapakse's Sri Lanka Freedom Party, said the outgoing president had a "very emotional" meeting with ministers as he bowed out on Friday.

- Sweeping reforms -

Sirisena has promised sweeping reforms of the presidency and said he will transfer many of its executive powers to parliament.

He was elected on a tide of resentment against Rajapakse, who rewrote the constitution after his re-election in 2010 to remove the two-term limit on the presidency and give himself more powers over public servants and judges.

During the campaign, Sirisena said that he had warned Rajapakse to change his ways or risk new unrest in the country.

"He was leading the country down a dangerous road to destruction," he had said, promising a "constitutional revolution" if elected.

Rajapakse enjoyed huge support among majority Sinhalese voters after overseeing the end of a separatist war by ethnic Tamil rebels in 2009.

But critics say he failed to bring about reconciliation in the years that followed his crushing victory over the Tamil Tiger guerrillas.

He is also accused of undermining the independence of the judiciary and has packed the government with relatives, sparking resentment even within his own party.

Rajapakse fell out with the West over allegations his troops killed 40,000 Tamil civilians at the end of the civil war, and refused to cooperate with a UN-mandated investigation.

He cultivated close links with China, which has invested heavily in Sri Lanka, seeking to counter rival regional power India's influence.

Beijing on Friday downplayed suggestions the new leadership could impact its projects in Sri Lanka.

The opposition has promised to address international concerns over war crimes and normalise relations with Western nations and India, whose Prime Minister Narendra Modi congratulated Sirisena.

- Celebrations in Colombo -

Firecrackers could be heard going off in Colombo as residents celebrated after Rajapakse's concession of defeat.

Sirisena's decision to run triggered a slew of defections and became a rallying point for disaffection with Rajapakse and his powerful family.

His vision for the country ties in closely with the free-market policies of the centre-right UNP which provided him with the political base to contest the election.

But analysts say he faces a challenge to unite the rainbow coalition of parties from right-wingers to Marxists that helped him secure victory.

The vote passed off largely peacefully, although there were some reports of intimidation in Tamil areas.

The president had come under international pressure, with Washington urging him to ensure peaceful and credible polls.

The election came days before a visit to the island by Pope Francis which some Catholic leaders had said should be cancelled in the event of violence.

Election monitors said large numbers of people had voted in the Tamil-dominated former war zones of the north and east, which are heavily militarised.

Tamils are Sri Lanka's largest minority, accounting for 13 percent of the population and helped bring down Rajapakse by supporting his rival.

"We voted to get our dignity back," said a Tamil journalist who asked not to be named.

"We may have good roads and a new railway line, but what we want is to live in peace."