KANO, Nigeria - Nigerian opposition leader Muhammadu Buhari claimed a historic victory Tuesday, defeating President Goodluck Jonathan in the country's first democratic ouster of a sitting leader.

With just one of 36 states left to deliver results, Buhari had 15.1 million votes to Jonathan's 11.7 million, according to an unofficial tally by the Reuters news agency. Jonathan called Buhari to concede late Tuesday afternoon.

"At about 5 minutes to 5, President Jonathan called Gen. Muhammadu Buhari, the winner of the elections, to congratulate him," opposition All Progressives Congress spokesman Lai Mohammed said.

Mohammed called Jonathan's action heroic. "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. Anyone who tries to foment trouble on the account that they have lost the election will be doing so purely on his own," Mohammed said.

The governing People's Democratic Party had questioned the results of Saturday's election in at least seven states, but ultimately conceded. Buhari's All Progressives Congress had unsuccessfully called for the cancellation of results in southeastern Rivers state, at the heart of Nigeria's crucial oil sector, after incidents of what it complained were intimidation and violence.

The opposition APC sent mass text messages to its supporters, urging them to remain indoors and celebrate with their families, warning that the PDP had hired thugs to shoot and bomb those celebrating.

Nigeria has had a troubled history of violently contested elections, political thuggery, vote rigging, underage voting, and contested elections. Observers watched tensely for Jonathan's response to the official results and that of disappointed PDP officials and supporters.

Jonathan's term was tainted by his failure to fight corruption and to deal decisively with the militant group Boko Haram, particularly when it abducted nearly 300 schoolgirls from a village in the country's troubled northwest. Jonathan launched strikes against Boko Haram late last year and scored a string of victories over the group, but it was too late to turn the tide of disenchantment.

A massive turnout in Buhari's stronghold, the impoverished and alienated north, which is predominantly Muslim, helped propel the opposition leader, himself a northern Muslim, and his All Progressives Congress. The turnout in Jonathan's strongholds in the south and southeast wasn't high enough to surpass Buhari's turnout in the populous north.

The dominance of the People's Democratic Party was undermined last year by a new political alliance, linking the north and southwest, and the desertion of many governors from the governing party to the APC.