Violence and technical glitches have affected polling as Nigerians turned out en masse to vote in what is expected to be one of the tightest presidential races in their history between incumbent Goodluck Jonathan and former military ruler Muhammadu Buhari.
Analysts are calling the poll a pivotal historical event for the young democracy. Jonathan’s People’s Democratic Party (PDP) has ruled Africa's most populous nation virtually unopposed for 16 years.
But it is possible he could lose to Buhari, who has contested three previous elections but never come close to victory before.
Violence affected areas in the country's restive northeast both prior to, and during, polling on Saturday, with the Associated Press news agency reporting that at least 39 people were killed by suspected Boko Haram fighters.
RELATED: Electronic accreditation frustrates Nigeria voters
Witnesses and officials told AP that 14 people, including Gombe state legislator Umaru Ali, died on Saturday in attacks on the towns of Biri and Dukku.
The news agency also cited residents of the town of Miringa as saying that members of the armed group torched people's homes before polls opened and then shot them as they tried to escape. Twenty-five people were reportedly killed. There were conflicting reports over the timing of the attack with AP saying it occurred early on Saturday, and Reuters news agency saying it occurred on Friday.
Both sides 'confident'
Polling will continue on Sunday in some areas where new machines largely failed to read voters' biometric cards, said Kayode Idowu, spokesman of the Independent National Electoral Commission (INEC). That includes some areas of Lagos, a megacity of 20 million and Nigeria's commercial capital on the Atlantic coast.
Most Nigerians expect a tight race. Insiders on both sides say that they are confident of victory, and a January poll by Afrobarometer put the parties neck and neck with 42 percent of the vote each.
Buhari’s opposition coalition, the All Progressives Congress (APC), has quickly gained popularity by presenting itself as the face of change for voters who have grown frustrated by the government’s record on corruption and its inability to defeat a dogged insurgency waged by the Boko Haram group.
Nigeria Decides: Read our special online coverage of the race here
Al Jazeera's Yvonne Ndege
If Nigerians are unhappy with their leaders, can they vote them out in a peaceful, free and transparent way?
That's the test of Saturday's Presidential and parliamentary elections in Africa's largest democracy.
If they do vote out President Goodluck Jonathan, and the ruling People's Democratic Party which has ruled Nigeria for 16 years, since the end of military rule, it would be the first peaceful handover of power from one democrat to another in the country's history. In this case - to the key presidential opposition candidate, Muhammadu Buhari of the All Progressives Congress.
More than $40m has been spent on these elections by the election commission. Close to 60 million people have collected voter cards to participate. And there are more than 120,000 polling stations around the country.
The winner in the presidential race needs 50% of the vote plus 1. And at least 25% of the vote in 2/3rds of the country's 36 states to win. And thousands of parliamentarian running for around 500 seats, a simple majority.
It's a colossal logistical challenge - in a country with porous borders and uneven infrastructure. But the voters we met at polling stations in Abuja were not put off.
People seem resolute in their determination to participate in this election, despite long hours waiting to be accredited, and even longer hours waiting to vote.
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Police said on Saturday that two car bombs exploded at two polling stations in south-central Enugu state but did not hurt voters. Two other car bombs were detonated at a primary school in Enugu, state police Commissioner Dan Bature said.
Fighters from Boko Haram, who were the cause of a six-week delay to the poll, waved guns and forced voters to abandon polling stations in three villages of northeast Gombe state, witnesses told AP.
But, despite the reported violence, voting across the huge nation was largely peaceful and good-natured into the early evening.
INEC's official website was hacked but was quickly secured, said officials who said the site holds no sensitive material.
"Struck by Nigerian Cyber Army!" was the message left at www.inecnigeria.com .
Technical problems also plagued a new system for reading voting cards with biometric card-reading machines, prompting the electoral commission to promise an extra day of voting for some polling stations.
Even the president was affected. Three newly-imported card readers failed to recognise the fingerprints of Jonathan and his wife. He returned two hours later and was accredited without the machine using visual identification.
Biometric cards are being used for the first time to discourage the kind of fraud that has marred previous votes.
Many frustrated voters told Al Jazeera of problems with the machines
"We have had 100% failure reading fingerprints", Kofar Fada, an INEC worker in the rural Rano local government area of Kano, a state in Nigeria's predominantly Muslim north, said.
But many others at polling stations across the country took the delays and technical problems in good spirits, one line of voters even bursting into song as they waited to cast their ballots.
"I am ready to cast my vote at whatever cost," said Tandalami Balami, who fled the town of Gwoza, recently taken back from Boko Haram by the military, to a camp in Maiduguri.
Electoral officials stressed that once voting starts it will not end until the last person in line has voted, even if it takes all night.
(With reporting by Eleanor Whitehead and Tom Saater in Kano)
Source: Al Jazeera and agencies