Former President Uhuru Kenyatta on Saturday joined poll observers in observing voting at various polling stations in Abuja.
Uhuru is in the country in his capacity as the leader of the African Union Election Observer Mission (AUEOM).
Voting in Nigeria's seventh General Election opened at 8.30am local time and was expected to close at 14.30 (13:30 GMT).
Voters cast ballots to elect the next President, Vice President and Members of the Senate and House of Representatives.
According to the country's Independent National Electoral Commission (INEC), there were more than 93.4 million eligible voters.
The BBC said that as at 12.40pm local time (11.40 GMT), voters at Amuwo Odofin in Lagos were still waiting to cast their votes as officials from INEC had not showed up.
Addressing the press,INEC chief Mahmood Yakubu said some delays were due to advice given by the security agencies for the safety of personnel and materials.
He said armed men had attacked some polling units in the southern state of Delta, where at least two voter card verification machines were carted away.
Yakubu said the stolen items were subsequently replaced and security boosted to allow voting to take place.
Other areas attacked included the northern state of Katsina,north-eastern state of Borno, where some electoral officers were reported to be injured during a shooting.
