The Sri Lankan president, Mahinda Rajapaksa has conceded defeat in an election he had been widely predicted to win before a member of his own party defected to run against him, the presidential office has said.

"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 news agency.

Maithripala 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.

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

"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."

Sirisena, a former government minister 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 perennially 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.

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

Source: Al Jazeera and agencies