The Latest: Nigeria's leader already claiming election win

The Latest on Nigeria's election (all times local):

8:25 a.m.

Nigeria's president says he will be congratulating himself at the end of the election after he was among the first Nigerians to cast their ballots.

A jovial President Muhammadu Buhari brushed aside reporters' questions about whether he would accept a loss to top challenger Atiku Abubakar in a race some observers now see as too close to call.

Buhari, voting in his northern hometown of Daura, jokingly checked the ballot his wife was casting to see whom she had voted for.

The president called the voting process smooth but in other parts of the country some officials were reporting concerns with a delayed opening of polls and a heavy security presence perhaps intimidating potential voters.

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8:15 a.m.

Nigerians have begun voting in a presidential election one week after a surprise last-minute delay blamed on logistical challenges.

President Muhammadu Buhari has cast his ballot as he seeks a second term in a race that observers now say is too close to call with top challenger and former Vice President Atiku Abubakar.

The ailing Buhari has been criticized for not delivering enough on his promises to tackle insecurity, the economy and corruption. Abubakar points to his business success in making sweeping pledges to turn the economy around but is dogged by corruption allegations.

Gunfire has been heard in at least two cities shortly before the polls opened, but police in Maiduguri in the northeast called the blasts there a show of force by security forces.

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7:55 a.m.

Multiple blasts in Nigeria's northeast are opening election day as President Muhammadu Buhari seeks a second term in Africa's most populous nation.

The blasts in the Borno state capital of Maiduguri came shortly before polls were to open. Police there say it was for "security purposes" and not an attack.

Gunfire also has been heard in parts of Port Harcourt in the restive south, where the military presence is said to be heavier than in past elections.

Buhari in a final address to the nation on Friday vowed that the more than 72 million Nigerians who can vote in this election would be able to go to the polls in peace.

But the Boko Haram extremist group, its Islamic State-affiliated offshoot and various agitators across the country have other plans.