Nigerian President Goodluck Jonathan has called his rival, Muhammadu Buhari, to concede the election, a Buhari adviser and other officials said, after election results on Tuesday showed Buhari with an insurmountable lead.
The Independent National Electoral Commission had not announced a winner as of Tuesday evening, though with only one state's results remaining, Buhari led Jonathan by nearly 2 million votes.
Buhari also won at least 25 percent of the vote in at least 24 states, as required under the constitution.
Yau Shehu Darazo, a top aide to Buhari, said Jonathan had called his rival to congratulate him. Officials from both Buhari's party and Jonathan's party also confirmed to VOA that Buhari had accepted Jonathan's concession.
The result marks the first time in Nigeria's history that an incumbent president was ousted at the ballot box and also heralds the end of a 16-year rule of Jonathan's Peoples Democratic Party.
Some victories in south
Some of Buhari's victories came in states in southern Nigeria, which is generally considered a stronghold for Jonathan, 57.
Buhari, 72, is from the north and served as the country's military ruler in 1984 and 1985 following a coup.
Tensions persisted in Port Harcourt, in the oil-rich Niger Delta region, where officials declared a dusk-to-dawn curfew. Rivers state has been the scene of some post-election protests.
The All Progressives Congress has called for the election there to be redone after alleged vote-rigging.
Jega, however, said Tuesday that commission investigators found no “substantial grounds” to cancel the vote there.
Post-election violence marred the country's disputed 2011 vote, resulting in about 800 deaths, mostly in the nation's northern regions.
On Monday, U.S. Secretary of State John Kerry and British Foreign Secretary Philip Hammond released a joint statement saying there are "disturbing indications" that the collation process "may be subject to deliberate political interference."
The U.S. and British governments "would be very concerned by any attempts to undermine the independence of the electoral commission or its chairman ... or in any way distort the expressed will of the Nigerian people," the statement said.
A spokesman for Jonathan's campaign called the allegations "absolute balderdash." Spokesman Femi Fani-Kayode challenged Kerry or any other foreign official to provide evidence.
International observer missions had called the conduct of the voting as "acceptable," despite technical glitches in a new computerized, anti-fraud system that prompted officials to extend voting by a day.
Nigerians across the country appeared to be closely watching the commission's work, broadcast live on national TV.
“There’s tension right now in Lagos because people are anxious to have the results. People are anxious," said David Olayemi, a Lagos resident. "They are waiting for the results, for the announcement of the new elected president."
In the northern city of Kano, where men sat drinking tea and listening to a radio, Abdul Rashid Sani said he was suspicious the government was up to something. He said the results from previous elections came faster, but he was confident that his party, the All Progressives Congress, will win.
In Kaduna, Al Amin Jinge also voiced concerns about the outcome.
“Everybody is afraid of the announcement of result. That’s why everybody hide at home," he said. "But we pray so nothing will happen, that, God willing, there is not any violence.”
Nigeria was originally due to hold the election in mid-February, but officials pushed back the vote because of fighting and instability in northeastern states where Boko Haram militants have been battling the government since 2009.
The security situation improved after a multinational offensive drove the militants from many towns they had controlled.
Also contributing was Chris Stein in Kano, Ibrahima Yakubu in Kaduna. Katarina Hoije in Lagos and Hilary Uguru in Port Harcourt. Some material from Reuters was used in this report.