Independent National Electoral Commission (INEC) began announcing state-by-state results from national elections on Sunday, amid complaints of irregularities, though it is not expected to name a victor in the race to succeed President Muhammadu Buhari for several days.
The presidential vote is expected to be the closest in Nigeria's history, with candidates from two parties that have alternated power since the end of army rule in 1999 facing an unusually strong challenge from a minor party nominee popular among young voters.
Votes in presidential and parliamentary elections are collated in each of Nigeria's 36 states before the count is transmitted to the electoral commission's central tallying centre in the capital Abuja.
The first results, from Ekiki state, showed a majority of votes for president cast in favour of Bola Tinubu of the governing All Progressives Congress.
Tinubu pulled in more than 200,000 votes in the state, against less than half that total for Atiku Abubakar of the main opposition PDP and just over 11,000 for Peter Obi of the Labour Party.
Independent National Electoral Commission (INEC) chairman Mahmood Yakubu adjourned the session following the first results and said the release of tallieswould resume at 11 a.m. (1000 GMT) on Monday.
However all three parties complained of irregularities. Obi's Labour party lambasted the electoral commission for failing to upload results directly from each polling unit to its website, as it had promised to do to guarantee transparency.
More than 24 hours after polls were meant to close, many polling stations have yet to upload their results. Nigerian Civil society organisations decried the error and the failure of several polling stations to open on time in a news conference.
INEC apologised for these glitches in a statement.
"The Commission is aware of challenges with the INEC Results Viewing Portal ... (which) has been relatively slow and unsteady," it said, blaming "technical hitches."
Both the PDP and APC complained of local INEC officials being pressured to alter results before submitting them.
There were also incidents of violence and intimidation, but seemingly not on the scale of previous elections.
Soldiers were forced to intervene on Sunday after 15 men pretending to be INEC officials stormed a voting collation centre in the Alimosho neighbourhood of Lagos, attacking party agents with knives and sticks, footage obtained by Reuters TV showed.
"Hoodlum boys ... just came and started coming out with daggers and hitting everybody that are Labour Party agents," the party's ward secretary Jacob Sulemain said, a masked soldier behind him. The footage showed some of the gang kneeling after being detained.
Voting had to be extended into Sunday in a few parts of the country after glitcheson Saturday, but counting has been underway since polls closed with the final tally expected within five days.
In one polling station in Yenagoa on Sunday, voters stood on sandy, weed-choked ground checking for their names plastered on a half-built concrete house. Voting also continued on Sunday in some parts of northeastern Borno state after voting machines failed to work.
It was not clear how many of Nigeria's 93 million registered voters were unable to cast a ballot on Saturday.
In most parts of the country of 200 million people, voting went smoothly.
There were reports of violence in the northern state of Kano on Sunday, where an armed group attacked a collation centre in the town of Takai before security forces arrived, said Rakiya Muhammad, an election observer who witnessed the incident.
Outgoing President Buhari, a retired army general who was also once a military ruler in the 1980s, is stepping down after winning two previous elections and serving the maximum eight years permitted by the constitution.
His successor will face a litany of crises gripping Africa's top oil producer and the continent's most populous nation.
Nigeria is struggling with Islamist insurgencies in the northeast; an epidemic of kidnappings for ransom; conflict between herders and farmers; shortages of cash, fuel and power; and deep-rooted corruption and poverty.
"I witnessed the worst experience of my life under this administration. Recently I spent two days without eating anything," said Ahmad Sulaiman, 49, who sells handbags in a market, as he stood in the baking sun in a dusty alleyway in Kano city.
"I voted because I wanted change," he added. He declined to say who he had voted for.