Millions of Nigerians have voted in a largely peaceful presidential election that is being heralded as one of the nation's most credible.
By Saturday evening, early results pointed to a close race between Goodluck Jonathan, the incumbent president, and Muhammadu Buhari, a former military ruler from the mostly-Muslim north.
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.
But there were some reports of irregularities, including an Associated Press report that said boys who appeared to be under 18 - the voting age in Nigeria - were seen queuing to vote.
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.
Early results showed President Jonathan, the first head of state from the southern oil-producing Niger Delta, and done well in the Christian south, including Lagos where his ruling party had struggled in last week's parliamentary election.
Opinion polls have generally given Jonathan the lead, but early results from the predominantly Muslim north showed a great deal of support for Buhari, which could help counterbalance his lack of support in the south.
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.
At least 17,000 security personnel, including soldiers and policemen, were deployed across Abuja, the nation's capital ahead of the vote, a senior police spokesman said.
Land borders were closed and a curfew imposed overnight.
Source: Al Jazeera and agencies