Early results in the Nigerian presidential polls show the incumbent, Goodluck Jonathan, leading in southern areas, while Muhammadu Buhari, the country's former military ruler, has attracted support from the country's Muslim north.
Results continue to trickle in from what has largely been a peaceful presidential election, though violence has been reported from the north, where Buhari's supporters have expressed anger over perceived vote rigging.
Early results showed 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.
First results from the heavily Muslim northern states showed Buhari with a wide lead and a high turnout which could outweigh his lack of support in the south.
"The election ... passed off quite peacefully yesterday. We know now that the ballot papers [from about 120,000 polling stations] ... are being collated at various centres across the country in some 36 Nigerian states," reported Al Jazeera's Yvonne Ndege from Abuja, the Nigerian capital, on Sunday.
"Many of those ballot papers will eventually end up ... in Abuja, where the chairman of the independent national electoral commission will tell Nigerians who the next president is.
"They had talked about ... results coming out [on Monday], but it now appears due to ... some kind of public pressure, there's momentum to try and get the results out today."
Analysts say they are heartened by the fact that the poll is being taken seriously, but concerned at the possible repercussions of a north-south divide emerging in the results.
"There's good news in this Nigerian presidential election: we're counting actual votes and people are interested in the count," said Chidi Odinkalu of the Open Society Justice Initiative NGO.
"And quite bad news: the country is badly divided, north vs. south."
Riots, bomb blast reported
Tensions ran high in the north as Buhari's followers feared an attempt to rig the vote count, and eventually boiled over in a number of incidents.
"There were riots in Misau by a mob which burnt down our party offices and an [electoral commission] office in the town over spurious allegations of electoral rigging," said Abdulmumini Kundak, an advisor to the state governor.
Trouble also flared in other parts of the north, with police investigating a bomb blast in a hotel near a polling station in Kaduna, a ruling party official's house being burned down in Azere, and shots being fired in Bauchi.
A mob also burnt down the house of a government official in Jada, in Adamawa state, after accusations that he had hidden ballot boxes there, Musa Daura, the state police commissioner said.
"The house was completely burnt. We've launched an investigation into the incident. No casualty was recorded," he said.
Daura said a similar incident occured in Mubi, also in Adamawa state, in addition to the earlier attack in Azere.
In Bauchi and Kano, the main city in the north, residets said that some voters slept outside ballot-counting centres, refusing to leave in order to make sure the results were not rigged.
In Jalingo, the capital of the northern Taraba state, police fired tear gas at a crowd that insisted on following the electoral commission's staff to a vote-collating centre to ensure results were not overturned.
"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.
A runoff between Jonathan and Buhari could risk polarising voters along regional lines in the country of 150 million, where ethnic and religious rivalries bubble near the surface.
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
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.
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.