Vote counting is underway in Nigeria’s tightest presidential election since the end of military rule in 1999.
Voting was marred by delays as polling stations failed to open in some areas due to logistical problems and security incidents.
The turnout was seen to be large, with many young voters arriving for the first time before dawn to vote.
The election is the largest 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 replace President Muhammadu Buhari – Peter Obi’s Labor Party, which is supported by many young people.
Tens of thousands of polling stations are counting the results, which will be collected and sent to the election headquarters in the capital Abuja.
Final results are not expected until Tuesday at the latest.
At a press conference, the head of the election, Mahmood Yakubu, apologized for the delay in voting, but said that everyone who queued at 13:30 GMT (14:30 local time) would be allowed to vote, even though polling stations had officially closed by then.
Voters in the largest city, Lagos, cheered as election officials arrived at the polling station on the outskirts of Lekki city almost four hours after the polls officially closed.
There were also reports of violence and seized ballot boxes in Lagos.
Although some voters were angry at the delay, others waited patiently to vote.
“As a Nigerian, you expect something to happen, so I went out with a power bank and a bottle of water. I will wait until they come so I can vote,” Edith, the first voter, told the BBC.
Mr Yakubu said gunmen had attacked several polling units in southern Delta state and northern Katsina state, where voter card verification machines were being transported.
They were later replaced and security was beefed up to make it possible, he said.
But voting was delayed until Sunday at 141 polling stations in the oil-rich southern state of Bayelsa due to disruption.
In northeastern Borno state, Mr. Yakubu said militant Islamists had set fire to election officials from a mountaintop in the Gwoza region, killing several officials.
The lead-up to the poll was overshadowed by a cash shortage caused by a botched attempt to redesign the currency, leading to widespread chaos in banks and cash machines as people desperately seek access to money.
New notes were introduced to deal with inflation, and also to buy votes. On the night of the House of Representatives election, he was seized with almost $500,000 (£419,000) in cash, and a list of people to whom he was due, police said.
Whoever wins will have to deal with a currency redesign, a collapsing economy, high youth unemployment, and widespread insecurity that left 10,000 people dead last year.
Voters also voted for 109 federal senators and 360 members of the house of representatives, with another vote for state governor in March.
The election has attracted the interest of young people – a third of eligible voters are under 35.
Mr Obi, 61, had hoped to break Nigeria’s two-party system after joining the Labor Party last May.
Although he was in the PDP before, he is seen as a relatively fresh face and enjoys strong support in some sections of the Nigerian youth, especially in the south.
The wealthy businessman was the governor of south-eastern Anambra State from 2006 to 2014. His supporters, known as “OBIDients”, say he is the only candidate with integrity, but critics argue that votes for him were wasted because he is unlikely to win.
However, the PDP, which ruled until 2015, wants Nigerians to vote for Atiku Abubakar, 76 – the only major candidate from the country’s Muslim-majority north.
He had been president five times before – all of them lost. He has been dogged by allegations of corruption and cronyism, which he denies.
Most of his career has been spent in the corridors of power, working as a civil servant, vice president and prominent businessman.
Most people see the election as a referendum on the APC, which has overseen a period of economic hardship and insecurity.
His candidate, Bola Tinubu, 70, is credited with building Nigeria’s commercial hub Lagos, during his two terms as governor until 2007.
He is known as the godfather of politics in the south-west region, where he wields great 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 must have the most votes and 25% of the votes in two-thirds of Nigeria’s 36 states to be declared the winner.
Otherwise, there will be a run-off in 21 days – a first in the history of Nigeria.