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.
US positive on Iran deal but talks still uncertain as ceasefire end nears
The United States expressed confidence that peace talks with Iran would go ahead in Pakistan and a senior Iranian official said Tehran was considering joining, but significant hurdles and uncertainty remained as the end of a ceasefire loomed.
Gunman kills one, wounds 13 at Mexico’s Teotihuacan pyramids
A gunman shot one dead as he opened fire at Mexico's Teotihuacan pyramids on Monday before taking his own life, authorities said, in a rare attack at a major tourist attraction.
Iran says no decision yet on joining peace talks
Iran is considering attending peace talks with the United States in Pakistan, a senior Iranian official told Reuters on Monday, following moves by Islamabad to end a US blockade of Iran's ports, a major hurdle for Iran to rejoin peace efforts.
Israeli strikes kill five in Gaza, say health officials
Israeli strikes have killed at least five Palestinians in separate incidents in the Gaza Strip on Monday, Palestinian health officials said, while fighters from Hamas clashed with gunmen from an Israeli-backed militia, witnesses said.
Trump says new deal with Iran will be better than old one
President Donald Trump said on Monday he believed a nuclear deal the US is currently negotiating with Iran will be better than a 2015 international agreement to curb Tehran's nuclear programme.
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