Atiku Abubakar challenges Muhammadu Buhari
Nigerians are finally going to the polls for the country's general election following a last-minute postponement of the vote a week ago.
The main challenger to President Muhammadu Buhari, 76, is the former vice president, Atiku Abubakar, 72.
Whoever wins will have to address power shortages, corruption, security threats, and a sluggish economy.
Voting began at 08:00 (07:00 GMT) and polls are due to close at 14:00 (13:00).
President Buhari cast his ballot in his hometown of Daura in the northern state of Katsina. Asked if he would congratulate his rival if he lost, he said: "I will congratulate myself."
About two hours before polls were due to open on Saturday, residents of the town of Maiduguri, the capital of Borno state in the country's north-east reported hearing several blasts and gunfire.
State police said in a statement that there was no threat to public safety. "The gunfire were not targeted at members of the public but was for security purposes," the statement said.
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Borno is the birthplace of the Islamist militant group Boko Haram, which had said it planned to disrupt the elections.
In the north-eastern state of Yobe, suspected Islamist militants attacked the town of Geidam, forcing people to flee, the Reuters news agency reported.
Why was the election delayed?
The initial vote was rescheduled in a dramatic press conference in the early hours of Saturday 16 February, just five hours before polls were due to have opened.