Incumbent candidate Goodluck Jonathan is heading for victory by a landslide margin in Nigeria's presidential election.
With most ballots counted in 35 of the West African country's 36 states Jonathan had tallied more than 22 million votes while his nearest rival Muhammadu Buhari, the country's former military ruler, had around 12 million.
Formal confirmation of the result is not expected before Monday, but the margin of Jonathan's lead suggests he has secured election without requiring a runoff vote. 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.
Al Jazeera's Yvonne Ndege, reporting from the capital, Abuja, said: "It's safe to say that the cat is out of the bag, and that Jonathan has won this election."
Jonathan polled strongly in predominantly Christian southern areas, including some of Nigeria's most populous cities, while Buhari attracted most of his support in the country's Muslim north.
Observers hailed the vote as "orderly and transparent" but some Buhari supporters claimed the results looked suspicious while youths in some northern cities set up burning barricades, the Reuters news agency reported.
In Lagos, Jonathan's People's Democratic Party (PDP) picked up 65.9 per cent of the vote, according to preliminary results from the Independent National Electoral Commission.
The PDP, which has won every presidential vote since Nigeria returned to civilian rule in 1999, had struggled there in last week's parliamentary election.
Oronto Douglas, a senior advisor to Jonathan, told Reuters: "This is no time for triumphalism. It is a time for deep reflection, for strengthening the bond of our union and for all of us to work together."
So far, there has been no official reaction from the candidates.
PDP offices attacked
More than 73 million people were registered to vote at 120,000 polling stations in Sunday's election.
Earlier on Sunday rioting was reported to have broken out in four of Nigeria's 36 states, with the offices of the PDP being burnt down in one state after some voters suspected the party of vote rigging.
"We're also told the official of the ruling party in another state had his home burnt down when some ballot boxes were found in his house," our correspondent said.
"In Kano state, not far from Abuja, we're hearing that many voters refused to leave the polling stations, wanting to see all the votes counted. That caused some minor skirmishes."
But in the state of Taraba, 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.
Our correspondent said that people speaking to Al Jazeera at polling stations were frustrated about living in a country that is the world's sixth-largest exporter of oil, but where many still remain without basic services, such as electricity and clean water.
"Corruption is also a major problem, and that's why people are acting and behaving so fervently, in terms of making sure that the vote is transparent, making sure it's free and fair," she said.
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," Chidi Odinkalu, of the Open Society Justice Initiative, a non-governmental organisation, said.
"And quite bad news: the country is badly divided, north vs south."
'Orderly and transparent'
Festus Mogae, a former president of Bostwana and head of the Commonwealth observer mission monitoring the vote, told Al Jazeera he was "very impressed" with what he had seen, and described the elections as "orderly and transparent and therefore a pleasant surprise given the fact that this country has been notorious for flawed elections."
He also said that the public has been "very, very patient and orderly".
"Of course, I'm crossing my fingers, because the polls are not totally out, not all of them, and there have been isolated reports of violence but ... at the vast majority of polling stations, polling went very well," Mogae said.
While international observers were positive about the elections, 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 centre 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.