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."
In Ogun: The moment Obasanjo leaned forward to check out the card reader... just before casting his vote.
MANUAL VOTING IN AKWA IBOM
In polling unit 12, ward 4, Ikot Obio Ndua, there is no card reader. The presiding officer said they are voting manually.
In Lagos: Bola Tinubu, APC chieftain, now voting in Alausa, Ikeja.
In Lagos: Governor Akinwunmi Ambode has just voted in Epe.
ELECTORAL MATERIALS HIJACKED IN IMO
Armed men in army and police uniforms have reportedly hijacked election materials in Aboh Mbaise local government area of Imo. They were said to have carted away the materials kept at Central School, Mbutu, before the electoral process began.
In Delta: Voters at PU010, Uwie LGA are complaining that the ballot paper is not stamped and signed by INEC.
In Lagos: Card reader has malfunctioned at ward 10, PU 003, Egbeda. Voters and INEC adhoc staff are waiting for a replacement.
In Edo: Adams Oshiomhole casts his vote, says the ongoing exercise is "an improvement" from the 2015 elections.
In Abuja: Impressive voter turnout at polling units 021 and 022 at the presidential villa. Accreditation/voting in progress.
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