Long delays have been reported at some polling stations in Nigeria as it holds its most competitive presidential election since military rule ended.
The electoral chief said security concerns and logistical problems led to delays in some areas.
Some polling stations have been attacked by armed men, who carted away voting machines, he added.
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 military rule ended in 1989.
But this time, there is also a strong challenge from a third-party candidate in the race to succeed President Muhammadu Buhari – the Labour Party’s Peter Obi, who is backed by many young people.
Some voters complained to the BBC that their polling stations had failed to open, two hours before they were due to close.
Voting machines malfunctioned in some areas, with voters told to return later.
There have been also been reports of violence and the snatching of ballot boxes in Lagos, a stronghold of the APC.
At a press briefing, the electoral chief, Mahmood Yakubu, apologised for the delays.
He gave an assurance that everyone who was in a queue by 13:30 GMT (14:30 local time) would be allowed to vote, even though polling stations are supposed to close by then.
Yakubu added that armed men had attacked some polling units in the southern state of Delta, where at least two voter card verification machines were carted away.
The stolen items were subsequently replaced and security boosted to allow voting to take place, he said.
In the northern state of Katsina, suspected criminal gangs had snatched at least six voter card checking devices, known as Bvas.
Three of them were recovered and the others had been replaced, the Inec boss said.
In the north-eastern state of Borno, where an Islamist insurgency has been waging for more than a decade, Yakubu confirmed that militants had opened fire on electoral officers from a mountain top in the Gwoza area.
Some electoral officers were reported to be injured in the shooting, he said.
The lead-up to the polls has been 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 the currency redesign, a crumbling economy, high youth unemployment, and widespread insecurity which saw 10,000 killed last year.
The vote for senator in the south-eastern Enugu East constituency was postponed after an opposition candidate was killed on Wednesday by suspected gunmen from the separatist group, Ipob.
The election has seen a huge interest from first-time voters and young people – a third of the 87 million eligible voters are below 35 – which may lead to a high voter turn-out than the 35% recorded in 2019.
“It is my responsibility and I have seen how important it is to vote,” 19-year-old first-time-voter Blessing Ememumodak told the BBC in Lagos.
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 Obi is wasted as he is unlikely to win.