- By Nduka Orjinmo
- BBC News, Abuja
The first results are expected later from the most tense elections in Nigeria since the end of military rule in 1999.
However, after widespread delays and some attacks on polling stations on Saturday, voting in parts of the country has been postponed until Sunday.
In other areas, voting went on all night.
Turnout seems high, especially among young people who make up about a third of the 87 million eligible voters.
This makes it the largest democratic exercise in Africa.
The elections presented an unprecedented challenge to the two-party system that has dominated Nigeria for 24 years.
Peter Obi of the relatively unknown Labor Party, Bola Tinubu of 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.
Saturday’s vote was marred by long delays at polling stations, as well as scattered reports of ballot theft and attacks by gunmen.
Dr. Nkem Okoli was about to vote in the Lekki district of the largest city of 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 officials’ phones. Now we can’t vote.”
In at least five states, voting did not begin in some places until around 6 p.m. local time — three and a half hours after the polls were due to close.
A woman on Twitter said she used her car’s headlights to illuminate the voting and counting process.
There is tension in parts of Rivers and Lagos states, where some political parties have asked their members to go to vote-gathering centers to avoid being manipulated.
There have also been complaints about the use of the recently introduced electronic voting system, with many voters accusing election officials of refusing to upload the results to polling stations as they should.
However, in those areas where voting went smoothly, the results are already posted outside individual polling stations.
The results of tens of thousands of polling stations across the country are then added together. An official from the electoral body in each of Nigeria’s 36 states will then travel to the capital, Abuja, where the state-by-state results will be announced. The first announcements are expected later on Sunday.
The final result is not expected until Monday at the earliest and possibly as late as Wednesday.
At a press briefing on Saturday, electoral chief Mahmood Yakubu apologized for the delay in the vote.
Mr Yakubu also said gunmen attacked some polling stations in southern Delta state and northern Katsina state, where voter card verification machines were taken away.
They were subsequently replaced and security increased to allow for voting, he added.
In the northeastern state of Borno, Mr Yakubu said militant Islamists had opened fire on election officials from a mountaintop in the Gwoza area, injuring a number of officials.
The run-up to the polls was overshadowed by a cash shortage caused by a failed attempt to redesign the currency, leading to widespread chaos at banks and ATMs as desperate people sought access to their money.
The new banknotes were introduced to deal with inflation, and also to buy votes. On the eve of the election, a member of the House of Representatives was arrested with nearly $500,000 (£419,000) in cash and a list of people to give it to, police say.
Whoever wins will face a crumbling economy, high youth unemployment and widespread insecurity, which killed 10,000 last year.
Voters also cast ballots for 109 federal senators and 360 members of the House of Representatives, with another vote for state governors in March.
Who are the main candidates?
Mr Obi, 61, enjoys staunch support among some sections of Nigerian youth, particularly in the south.
Although he was in the PDP before then, he is seen as a relatively fresh face. The wealthy businessman served as governor of the southeastern state of Anambra from 2006 to 2014. His backers, known as the “OBIDients”, say he is the only candidate with integrity, but his critics say a vote for him is wasted because one of the two traditional parties is more likely to win.
Instead, the PDP, which ruled until 2015, wants Nigerians to vote for 76-year-old Abubakar, the only major candidate from the country’s predominantly Muslim north.
He has run for president five times before – all of which he has lost. He is dogged by allegations of corruption and favouritism, which he denies.
He has spent most of his career in the corridors of power, serving as a top government official, vice president and a prominent businessman.
Most people view the election as a referendum on the APC, which has seen a period of economic hardship and growing insecurity.
The candidate, 70-year-old Tinubu, is credited with building Lagos, Nigeria’s commercial hub, during his two terms as governor until 2007.
He is known as a political godfather in the southwestern region where he wields enormous influence, but like Abubakar, he is also dogged by allegations of corruption over the years and ill health, both of which he denies.
A candidate must receive the most votes and 25% of the votes cast in two-thirds of Nigeria’s 36 states to be declared the winner.
Otherwise, a second round will follow in 21 days – a first in Nigerian history.
Additional reporting from BBC teams around the country.