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.
Five dead as Mexican plane crashes off Texas coast
At least five people died when a small Mexican Navy aircraft crashed on Monday off the Texas coast near Galveston during a mission for an organisation that helps child burn victims, including transferring them for treatment in the US.
Two killed in underground explosion at Polish coal mine
Two workers have been killed due to an outburst of methane and rock masses at the Pniowek coal mine in southern Poland, owner JSW said on Monday evening.
Netanyahu to discuss Iran, next phase of Gaza plan with Trump
Prime Minister Benjamin Netanyahu has said on Monday he will discuss Iran's nuclear activities during his visit next week with US President Donald Trump.
US President unveils 'Trump-class' battleships
US President Trump announced plans for a new "Trump class" of battleships on Monday, marking the start of an expanded naval buildup and signaling increased scrutiny of defence contractors over production delays and cost overruns.
Cambodia, Thailand to hold talks on resuming Trump's truce
Thailand and Cambodia have agreed on Monday to hold a December 24 meeting of defence officials towards resuming a months-old ceasefire, as fierce border fighting between them entered a third week with at least 80 people killed so far.
