Citizens are going to the polls today after a last-minute postponement earlier this week.
The main challenger to reigning president Muhammadu Buhari is former Vice-President Atiku Abubakar.
It is not known exactly when the final results will be announced.
Experts say a winner could be declared as late as Tuesday.
Nigeria is Africa's most populated country.

Shooting at Australia's Bondi Beach kills 12
Police hold person of interest after Brown University shooting leaves two dead
Hamas says Israel's killing of senior commander threatens ceasefire
Ukraine's Zelenskyy ditches NATO ambition ahead of peace talks
Thailand declares curfew along coast as Cambodia border fighting spreads
India tightens pollution curbs as Delhi's air quality worsens
'Peace is not far away' says Erdogan after Putin meeting
Belarus frees Nobel winner, protest figures as US lifts more sanctions
