The election was the most competitive presidential race ever in Nigeria, one of the largest democracies in the world. Now, if power is handed over peacefully, it will be a major shift for the nation — the first transfer of power between civilians of different parties in a country that has spent much of its post-colonial history roiled by military coups.
With all but one of Nigeria's 36 states counted, the former military ruler, Muhammadu Buhari, held a lead of more than two million votes over Goodluck Jonathan.
Mr. Buhari swept critical competitive states in the country's south-west. A belated convert to democracy, Mr. Buhari also piled up large vote totals, as expected, in his northern stronghold, crushing the incumbent in Kano, Nigeria's second-largest city.
The remaining state, Borno, is in the north, where Mr. Buhari enjoys broad support and the government has been widely condemned for allowing the Boko Haram militant group to sweep through villages and towns, killing thousands of civilians.
Since the end of military rule in 1999, Nigeria has been governed by a single, dominant party — Mr. Jonathan's Peoples Democratic Party (PDP).
"At about 5 minutes to 5, President Jonathan called General Muhammadu Buhari, the winner of the elections, to congratulate him," APC spokesman Lai Mohammed told Reuters.
"I think he conceded defeat. 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," he added.
"Anyone who tries to foment trouble on the account that they have lost the election will be doing so purely on his own."