Nigeria has continued counting ballots in a closely fought general election after failures in controversial new technology pushed voting into a second day, with President Goodluck Jonathan facing a stiff challenge from ex-military ruler Muhammadu Buhari.
Despite violence linked to Boko Haram and sporadic unrest elsewhere, UN chief Ban Ki-moon and others praised the conduct of the vote on Sunday and called for calm to avoid a repeat of deadly rioting that followed 2011 elections.
Voters who were unable to cast their ballot because of technical problems on Saturday returned to the polls on Sunday, as the main parties traded accusations over the use of the technology.
The country's electoral commission said some 300 of the 150,000 polling stations would be open on Sunday to accredit voters after handheld devices to read biometric identity cards failed.
Thousands of people demonstrated in the Rivers state, as the head of electoral commission said he was concerned about allegations of irregularities there.
Al Jazeera's Yvonne Ndege
If Nigerians are unhappy with their leaders, can they vote them out in a peaceful, free and transparent way?
That's the test of Saturday's Presidential and parliamentary elections in Africa's largest democracy.
If they do vote out President Goodluck Jonathan, and the ruling People's Democratic Party which has ruled Nigeria for 16 years, since the end of military rule, it would be the first peaceful handover of power from one democrat to another in the country's history. In this case - to the key presidential opposition candidate, Muhammadu Buhari of the All Progressives Congress.
More than $40m has been spent on these elections by the election commission. Close to 60 million people have collected voter cards to participate. And there are more than 120,000 polling stations around the country.
The winner in the presidential race needs 50% of the vote plus 1. And at least 25% of the vote in 2/3rds of the country's 36 states to win. And thousands of parliamentarian running for around 500 seats, a simple majority.
It's a colossal logistical challenge - in a country with porous borders and uneven infrastructure. But the voters we met at polling stations in Abuja were not put off.
People seem resolute in their determination to participate in this election, despite long hours waiting to be accredited, and even longer hours waiting to vote.
Follow Yvonne Ndege for the latest updates on Nigeria's elections @YvonneNdeg
Attahiru Jega, the chairman of the Independent National Electoral Commission (INEC), said that the problems were limited to "only about 450" card readers.
Jega told a news conference in the capital Abuja on Sunday that he was concerned about allegations and the electoral authorities were investigating the complaints, which centred on opposition party agents being excluded from a vote-tallying meeting.
RELATED: Electronic accreditation frustrates Nigeria voters
"The opposition says what is happening in the Rivers state is the beginning of the ruling party trying to steal the elections," said Al Jazeera’s Yvonne Ndege, reporting from Abuja.
Thousands took the streets of the state capital of Port Harcourt to protest alleged killings of opposition campaign workers and voting irregularities.
Police reported the electoral commission's office in Port Harcourt was bombed on Sunday, and that three people, including a soldier, were shot dead on Saturday.
However, the opposition coalition said scores of its members have been killed and blamed "ethnic militias" working for Jonathan's party.
On Saturday, many voters had camped overnight or arrived early at polling stations to have their credentials checked before returning to vote in the afternoon and long into the night.
Boko Haram loomed large, seeking to disrupt what it sees as the "un-Islamic" elections by launching a series of attacks.
Violence affected areas in the country's restive northeast both prior to, and during, polling on Saturday, with the Associated Press news agency reporting that at least 41 people were killed by suspected Boko Haram fighters in various parts of the country.
PROFILES: Goodluck Jonathan and Muhammadu Buhari
Jonathan, who became president in 2010 after succeeding Umaru Yaradua who died in office before completing his term, is contesting his second election. He comes from the predominantly Christian south while his challenger Buhari, who seized power in the 1980s, is from the mainly Muslim north.
Jonathan’s party has ruled Africa's most populous nation virtually unopposed for 16 years.
However, it is possible he could lose to Buhari, who has contested three previous elections but never come close to victory before.
Source: Al Jazeera and agencies