Nigeria counts votes in closely-fought presidential election even as voting continues in some units where polling opened late or ballot machines malfunctioned..
Security sources also reported attacks in Auno, 25 kilometres (15 miles) west of the city, and in Geidam, in neighbouring Yobe state.
Boko Haram's insurgency in northeast Nigeria has killed more than 27,000 people and left 1.8 million homeless, creating a humanitarian crisis.
According to Sahara Reporters, residents of Maiduguri are now in panic as series of explosions have rocked the city all within ten minutes.
Festus Okoye, the national commissioner for information and voter education at the Independent National Electoral Commission (INEC), says a vote will be considered void if the ballot paper is put in the wrong box. Voting turnout appeared to be light as authorities tried to calm panicked, skeptical residents.
Buhari, among the country's first voters, said he will be congratulating himself at the end of the election, jovially brushing aside reporters' questions about whether he would accept a loss to top challenger Atiku Abubakar in a race some observers see as too close to call.
" I have been voting since 1999".
Buhari was elected president in 2015, but many Nigerians believe he has failed to live up to his initial campaign promises which included cracking down on rampant corruption and fighting Boko Haram.
McCabe says Hill leaders didn't object when informed of Trump investigation
She predicted Trump would be reelected to a second term, despite his legal troubles. "Hey folks, Donald Trump's the president". McCabe has alleged that his firing was politically motivated - retaliation, he claims, for his opening the case against Trump.
He voted in his hometown of Daura, in northwest Nigeria.
It marked the third time in row that a vote has been delayed in Nigeria.
Polls closed in many areas, although voting continued in units that opened late.
The wife of the president looks to have allowed him so to do with the entire episode drawing laughter among the people present at the booth.
Buhari has again vowed to be tough on insecurity and corruption, and wants to complete much-needed road and rail infrastructure projects, as well as social mobility schemes.
Mahmood Yakubu, chairman of the INEC, said that 72.8 million cards had been collected out of 84 million registered voters.
Acting Inspector General of Police Mohammed Adamu said police would ensure security at the polls.
Gunfire also was heard in parts of Port Harcourt in Nigeria's restive south, where the military presence was said to be heavier than in past elections.
But the Boko Haram extremist group, its Islamic State-affiliated offshoot and various agitators across the country have other plans.