Nigerians are going to the polls to elect a president, with incumbent Goodluck Jonathan facing a strong challenge from Muhammadu Buhari.
Delays and technical problems were reported at the start of the election, said to be the most closely fought since independence.
It was postponed from mid-February to allow the army to recapture territory from Islamist Boko Haram militants.
The two main candidates have pledged to prevent violence.
By mid-morning, only 68% of polling stations were open, according to the Transitional Monitoring Group (TMG), the largest body observing the elections.
Voters need to register first using biometric cards with their fingerprints before they can cast their vote later.
At some polling stations, card readers appear to be working slowly or not at all, BBC reporters on the ground say.
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.”
However, Gen 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.
As millions of voters queued outside polling stations, the Independent National Electoral Commission (Inec) had its website hacked.
A group calling itself the Nigeria Cyber Army has placed a message on the website warning Inec not to rig the elections.
It is not clear if the security breach goes beyond the website, although the hackers say they have long been in control of the electoral commission database.
Inec says it is investigating the attack.
The election comes amid heightened security, with police deployed at all polling stations.
Suspected Boko Haram gunmen have killed two people at a polling station in Gombe state, AFP reports.
The authorities are reported to have carried out a controlled explosion after a car bomb was discovered at a polling station in Enugu state in south-eastern Nigeria.
There are also reports of election monitors being chased away from polling stations. A TMG spokesman told the BBC that groups of youths and party agents had been intimidating observers in three different locations in the south of the country.
