Early results point to a close race between incumbent President Jonathan and former military ruler Buhari.
Nigerian President Goodluck Jonathan looks set for a close race against ex-military ruler Muhammadu Buhari as results trickled in from a largely peaceful presidential election.
Early results showed Goodluck Jonathan had done well in much of predominantly Christian southern Nigeria, including areas such as the most populous city of Lagos, where his ruling party had struggled in a parliamentary election a week ago.
But first results from heavily Muslim northern states showed Muhammadu Buhari with a wide lead and a high turnout which could outweigh his lack of support in the south. Tensions ran high in the north as Buhari's followers feared an attempt to rig the vote count.
"Across the country it will be close," Nasir el-Rufai, a former government minister and Buhari supporter, told the Reuters news agency at a vote counting centre in the capital Abuja.
"My only fear is it will become a north-south issue if we see a situation where Buhari sweeps the north and Jonathan does well in the south. We may have to go to a runoff," he said.
To win in the first round, a candidate needs a simple majority and a quarter of the vote in two thirds of the 36 states. There are more than 73 million registered voters and 120,000 polling stations. Final results could take days.
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International observers were positive about the elections, which saw only limited evidence of the violence and vote rigging that have marred previous polls.
Joe Clark, a former Canadian prime minister who is observing the vote, said there would likely be problems but that overall Nigerian officials had gone to great lengths to ensure the elections ran smoothly.
"Some of the problems that existed in the immediately past election [last week's parliamentary vote] seem to have been addressed - people who were left off the voters list before and were unable to vote, a lot of that has been repaired," he told Al Jazeera.
"Coming from people we speak to in the polling line, the message is that they believe this will be an election where their vote will count, which has not been the case in the past."
He said that many of the electoral officials who use to run the process had been replaced with members of the National Youth Service Corps.
"These are young people, they are well-respected ... there is no question at all about their integrity," he said.
He also noted that the elections were run so that voters were present "regularly" at the polls, and so limiting the amount of time that ballot boxes were left out of the sight of civilian observers.
Reports of irregularities
The chief European Union observer said most stations in Africa's most populous nation opened on time, and that observers only saw a few cases of missing voting materials.
|"They [PDP] did less well than expected in the legislative elections last week but they still have plurality and clout which no other party has across the country"|
Nii Akuetteh, former executive director of Africa Action
Elsewhere, party officials helped people ink their fingers and mark their ballots.
One party worker accompanied an elderly woman to drop off her ballot in the box despite regulations banning party workers from voting stations.
And at one collation center in Lagos, volunteers carried blank ballots without supervision from election officials.
Security forces were on high alert after an explosion at a police station in Maiduguri, in the country's northeast, early on Saturday before the polls were due to open - the second such attack in 24 hours.
The presidential polls follow last week's parliamentary election, which had been repeatedly delayed by organisational issues but which were seen as a major step forward for the country's democracy.
Other candidates in the vote include former anti-corruption chief Nuhu Ribadu and Kano state governor Ibrahim Shekarau, but they are considered rank outsiders.
Speaking to Al Jazeera from Washington, Nii Akuetteh, a former executive director of Africa Action, said that he expected Jonathan to win.
"I will be extremely surprised if there is an upset because he is the incumbent and is the candidate of the powerful ruling party," he said.
"They [PDP] did less well than expected in the legislative elections last week but they still have plurality and clout which no other party has across the country."
Jonathan's PDP has won every presidential vote since Nigeria returned to civilian rule in 1999.