Elections in parts of Nigeria have been extended until Sunday after delays and a number of attacks.
An election official told the BBC the delays were "not widespread" but were still "a matter of concern".
Technical problems with new biometric cards slowed down voter registration, while more than a dozen people have reportedly been killed in various attacks by unknown gunmen.
Incumbent Goodluck Jonathan is facing a strong challenge from Muhammadu Buhari.
The election is said to be the most closely fought since independence.
It was postponed from mid-February to allow the army to recapture territory from the Islamist militants of Boko Haram.
The two main candidates had pledged to prevent violence during and in the aftermath of the elections.
Several hours after voting started reports came in of violent incidents at polling stations in which at least 13 are reported to have been killed.
Voters are also electing members of the house of representatives and the senate.
By 12:30 local time (11:30 GMT), only 81% of polling stations were open, according to the Transitional Monitoring Group (TMG), the largest body observing the elections.
This was 30 minutes before the deadline to be accredited to vote.
Voters need to register first using biometric cards with their fingerprints before they can cast their vote.
However, at some polling stations, card readers are working slowly or not at all.
President Jonathan tried for some 50 minutes to register in his home village of Otuoke. A BBC reporter at the scene says he had to come back a second time and when the electronic registration failed again, he had to be accredited manually.
Problems were also reported from the north's biggest city of Kano, where thousands of voters waited for election officials and voting materials to arrive.
"We've been here since six o'clock and now it's half-past nine," Ismail Omar, a 65-year-old builder told AFP news agency.
"No-one has shown up from Inec (the Independent National Electoral Commission)… This is a deliberate attempt to sabotage the elections."
But General Buhari registered in his hometown Daura without any problems and he praised the accreditation system.
"If people are allowed to vote, rigging will actually be impossible under this system," he said.