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.