Early results have started to arrive from Nigeria's tightest election since the end of military rule in 1999.
Official results from the southwestern Ekiti state show a clear victory for ruling party candidate Bola Tinubu in one of his strongholds.
Further results will not be formally announced until 10:00 GMT on Monday.
Following widespread delays and attacks on some polling stations on Saturday, voting was postponed until Sunday in parts of the country.
Voting continued through the night in some areas.
Turnout appears to be high, especially among young people who make up about a third of the 87 million eligible voters.
This makes it the biggest democratic exercise in Africa.
The election has seen an unprecedented challenge to the two-party system that has dominated Nigeria for 24 years.
Peter Obi from the previously little known Labour Party, Mr Tinubu from the ruling All Progressives Congress (APC) and Atiku Abubakar of the main opposition Peoples Democratic Party (PDP) are all seen as potential winners. There are 15 other presidential candidates.
A candidate needs to have the most votes and 25% of ballots cast in two-thirds of Nigeria's 36 states to be declared the winner.
Otherwise, there will be a run-off within 21 days - a first in Nigeria's history.
Saturday's voting was marred by long delays at polling stations, as well as scattered reports of ballot-box snatching and attacks by armed men, especially in southern areas, where Mr Obi has strong backing.
Dr Nkem Okoli was just about to vote in the Lekki district of the biggest city Lagos when masked men attacked the polling station.
"There was pandemonium. There were bottles flying everywhere," she told the BBC. "They broke [the ballot box]. They stole the phones of the officials. Now we can't vote."
In some areas, voting did not begin until around 18:00 local time - three-and-a-half hours after polls were due to close.
First-time voter Susan Ekpoh told the BBC that she spent 13 hours at her polling station in the capital, Abuja, only leaving at midnight.
She said when it got dark, election officials said they needed light to see what they were doing, so she and others used their car headlights to illuminate proceedings.
The southern Bayelsa state was among those areas where voting was delayed until Sunday - it is not clear how many parts of the country saw voting postponed.
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