Maithripala Sirisena is expected to be sworn in as Sri Lanka's new president, an opposition spokesman said, after the incumbent, Mahinda Rajapaksa, conceded defeat in a bitterly contested election.

"The swearing in will be at 6:00pm local time today at Independence Square" in Colombo, spokesman Saman Athaudahetti told AFP news agency on Friday.

Sirisena thanked outgoing leader Rajapakse for allowing a "fair" election that officials described as one of the most peaceful in the island's history.

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

Earlier, the longtime Sri Lankan president conceded defeat in an election he had been widely predicted to win before Sirisena, a member of his own party, defected to run against him. 

"The president met with former prime minister Ranil Wickremesinghe this morning. The president concedes defeat and will ensure a smooth transition of power bowing to the wishes of the people," Rajapaksa's press secretary, Vijayananda Herath, told AFP. 


Related: Challenging Sri Lanka's 'war' president 


Sirisena, a former health minister and political ally of Rajapaksa, took an early lead in Thursday's election count, with the Department of Elections saying he had 56.5 percent of the initial votes counted, compared to 42 percent collected by the incumbent.

Al Jazeera's Charles Stratford, reporting from Colombo, said no-one had expected the margin to be so great.

MAITHRIPALA SIRISENA

Background 63-years-old

Former health minister in Rajapaksa's government

The son of a World War II veteran

Escaped assassination attempts from Tamil rebels at least five times  

Promises To reform the presidency within 100 days

To return the country to a parliamentary democracy where the police, the judiciary, and the civil service are independent institutions

Economic outlook Free-market, investor-friendly policies 

"It is frankly incredible news from Sri Lanka this morning, the country waking up to a new political leader.

Constitutional reforms

"It seems as if they have voted for political change in this country that has seen a leader lead this country for more than ten years," our correspondent said.

Sirisena, who deserted the president and changed sides to become the opposition's candidate in November, has vowed to root out corruption and bring constitutional reforms to weaken the power of the presidency.

Rajapaksa won handsomely in the last election in 2010, surfing a wave of popularity that sprang from the defeat in the previous year of ethnic Tamil separatists who had waged a crippling war against the state for decades.

But critics say he became increasingly authoritarian since becoming president, with several members of his family holding key positions of power.

Despite his waning popularity, Rajapaksa called the latest election early, confident that the fractured opposition would fail to find a credible challenger.

He did not anticipate the emergence of Sirisena, who dined with the president one night and turned on him the next day.

Minority Tamils are believed to have voted heavily against Rajapaksa after he ignored their demands to heal the wounds of the fighting and years of ethnic divisions.

Muslims, the second-largest ethnic minority, also appear to have voted against the former president, who was accused of turning a blind eye to attacks on Muslims and other minorities by far-right Buddhist groups.

And for the country's Sinhalese, which make up about three-quarters of the population, Sirisena's entry into the race gave them another credible option amid Rajapaksa's growing authoritarianism and move towards dynasty politics.  

Election officials said the turnout from an electorate of about 15 million was provisionally 65-80 percent.

Source: Al Jazeera and agencies