Former military ruler Muhammadu Buhari has become the first Nigerian to defeat a sitting president through the ballot box, putting him in charge of Africa's most populous nation and its biggest economy.

Al Jazeera's Yvonne Ndege, reporting from the capital Abuja, said Buhari was declared the winner after he gained 2.7 million more votes than his rival, incumbent President Goodluck Jonathan.

Our correspondent said that Buhari managed to secure more than 25 percent of votes in 24 states, ruling out a run-off vote.

To win the election, Buhari had needed more than 50 percent of the total votes nationally - and take at least 25 percent of the vote in two thirds of the states.

Jonathan has publicly conceded defeat and conveyed his "best wishes" to the president-elect.

He urged his supporters to follow "due process" in channeling their frustrations at losing the election amid fear of violence.

"Nobody's ambition is worth the blood of any Nigerian," he said in a statement issued after his election defeat. "The unity, stability and progress of our dear country is more important than anything else."

The All Progressives Congress (APC) party earlier said that Buhari had received a phone call from incumbent Jonathan during which he immediately conceded defeat and congratulated him on his victory.

"There had always been this fear that he might not want to concede, but he will remain a hero for this move. The tension will go down dramatically," Lai Mohammed, spokesman for the APC, said.

Victory for Buhari marks the first time in Nigeria's history that an opposition party has democratically taken control of the country from the ruling party.

Al Jazeera's Haru Mutasa, reporting from Lagos, said there was shock that Jonathan had congratulated Buhari and that violence had not followed the announcement. In the 2011 election, more than 800 people were killed in protests after Buhari was defeated by Jonathan.

"The announcement has been greeted with celebrations across the country," Mutasa said. "Many people are excited and hope this will mark a new beginning and move the country forward."

Hundreds of Buhari's supporters gathered to celebrate outside his home in Abuja, with some brandishing brooms to symbolize his promise to clean up corruption.

His supporters told Al Jazeera that the vote was "free, fair and without irregularities" as the country ushered in a new era.

"We don't have roads, electricity and the youth are looking for jobs," one supporter said. "The people wanted change and change has now come."

Jonathan, whose five years in office have been plagued by corruption scandals and an insurgency by the Boko Haram group, was trailing by around 500,000 votes before votes in pro-opposition areas were counted.

There was a brief protest by Jonathan's Peoples Democratic Party (PDP) before the counting had resumed on Tuesday.

Former Niger Delta minister Godsday Orubebe accused elections chief Attahiru Jega of being "partial" and "selective".

Orubebe claimed Jega had refused to investigate PDP complaints about big wins by Buhari in northern states but had launched a probe into claims by the APC of irregularities in Rivers.

Jega said later: "I don't believe that the allegations are substantial enough to require the cancellation or rescheduling of the elections in Rivers state. We will take the results."

International observers gave broadly positive reactions to the conduct of the vote, despite late delivery of election materials and technical glitches with new voter authentication devices.

Nigeria's Transition Monitoring Group, which had observers across the country, said: "These issues did not systematically disadvantage any candidate or party."

PHOTO CAPTION

Buhari supporters celebrate in Kano [Reuters]

Al-Jazeera