Vote counting is under way in Nigeria's tightest presidential election since military rule ended in 1999.
Turnout appeared to be high, with many young, first-time voters arriving before dawn to cast their ballots.
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.
Voting continued throughout the night in some areas, and was postponed until Sunday in parts of Bayelsa state.
In at least five states, voting in some places did not begin until around 18:00 local time - one-and-a-half hours after polls were due to close.
There is tension in parts of Rivers and Lagos states, where some political parties have asked their members to go to the centres where votes are being collated, over fears that they are being manipulated.
There have also been complaints over the use of the recently introduced electronic voting system with many voters accusing electoral officials of refusing to upload the results at the polling units as they are supposed to.
However, in those areas where voting went smoothly, results are already being posted outside individual polling stations.
The elections are the biggest democratic exercise in Africa, with 87 million people eligible to vote.
Politics has been dominated by two parties - the ruling APC and the PDP - since the restoration of multi-party democracy 24 years ago.
But this time, there is also a strong challenge from a third-party candidate in the race to succeed President Muhammadu Buhari - from the Labour Party's Peter Obi, who is backed by many young people.
The APC is represented by former Lagos state governor Bola Tinubu, while former Vice-President Atiku Abubakar is running for the PDP.
The results from tens of thousands of polling stations around the country are being collated and sent to the electoral headquarters in the capital Abuja.
The final result is not expected until at least Tuesday.
At a press briefing on Saturday, electoral chief Mahmood Yakubu apologised for the delays in voting.
The election day was largely peaceful, but there have been reports from Lagos of violence and ballot boxes being snatched. Some voters complained of being attacked and chased away from polling stations.
In other places, people reported being asked to either vote for a particular candidate or leave.
Mr Yakubu said that armed men had also attacked some polling units in the southern state of Delta and the northern state of Katsina, where voter-card verification machines were carted away.
They were subsequently replaced and security boosted to allow voting to take place, he added.
In the north-eastern state of Borno, Mr Yakubu said that militant Islamists had opened fire on electoral officers from a mountain top in the Gwoza area, injuring a number of officials.
The lead-up to the polls was overshadowed by a cash shortage caused by a botched attempt to redesign the currency, leading to widespread chaos at banks and cash machines as desperate people sought access to their money.
The new notes were introduced in order to tackle inflation, and also vote-buying. On the eve of the election a member of the House of Representatives was arrested with almost $500,000 (£419,000) in cash, and a list of people he was supposed to give it to, police say.
Whoever wins will have to deal with a crumbling economy, high youth unemployment, and widespread insecurity which saw 10,000 killed last year.
Voters also cast their ballots for 109 federal senators and 360 members of the house of representatives, with another vote for state governors in March.
The election has seen a huge interest from young people - a third of eligible voters are below 35.
Mr Obi, 61, is hoping to break up Nigeria's two-party system after joining the Labour Party last May.
Although he was in the PDP before then, he is seen as a relatively fresh face and enjoys fervent support among some sections of Nigeria's youth, especially in the south.
The wealthy businessman served as governor of the south-eastern Anambra State from 2006 to 2014. His backers, known as the "OBIdients", say he is the only candidate with integrity, but his critics argue that a vote for him is wasted as he is unlikely to win.
Instead, the PDP, which ruled until 2015, wants Nigerians to vote for Mr Abubakar, 76 - the only major candidate from the country's mainly Muslim north.
He has run for the presidency five times before - all of which he has lost. He has been dogged by accusations of corruption and cronyism, which he denies.
Most of his career has been spent in the corridors of power, having worked as a top civil servant, vice-president and a prominent businessman.
Most people consider the election a referendum on the APC, which has overseen a period of economic hardship and worsening insecurity.
Its candidate, Mr Tinubu, 70, is credited with building Nigeria's commercial hub Lagos, during his two terms as governor until 2007.
He is known as a political godfather in the south-west region, where he wields huge influence, but like Mr Abubakar, has also been dogged by allegations of corruption over the years and poor health, both of which he denies.
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.