Sri Lanka's newly elected president Mithripala Sirisena waves as he leaves the election commission office in Colombo on Friday.

AFP/Colombo

Maithripala Sirisena will be sworn in as Sri Lanka's new president on Friday after the strongman incumbent conceded defeat in a bitterly fought election, brought down by charges of corruption and growing authoritarianism.

A top aide to Mahinda Rajapakse said the outgoing president accepted the decision of voters who turned out in force on Thursday, in a remarkable turnaround 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, adding he had already vacated his main official residence in a symbolic gesture of defeat.

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% share of the vote to Rajapakse's 47.58%.

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 6pm (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, adding Rajapakse and Sirisena had spoken by phone.

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.

"We were very sad. Most of the ministers and others were in tears," said Thilanga Sumathipala.

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, becoming increasingly close to China.

The opposition has promised to address international concerns over war crimes and normalise relations with Western nations as well as regional superpower India.

Indian Prime Minister Narendra Modi said he had spoken to Sirisena to congratulate him.

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.

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

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.