Nigeria has delayed its presidential and parliamentary elections for a week, in a dramatic night-time move.
The Independent National Electoral Commission (INEC) made the announcement just five hours before the polls were due to open on Saturday.
"Proceeding with the election as scheduled is no longer feasible," commission chairman Mahmood Yakubu said, citing logistical issues.
The two main candidates have asked people to remain calm and be patient.
Mr Yakubu said the difficult decision to postpone was needed to ensure a free and fair vote.
The presidential and parliamentary votes have been rescheduled for Saturday 23 February.
Governorship, state assembly and federal area council elections have been rescheduled until Saturday 9 March.
The announcement came after an emergency meeting at the INEC headquarters in the capital, Abuja.
What's the reaction?
Nigeria's two main political parties, the ruling All Progressives Congress (APC) and the People's Democratic Party (PDP), swiftly condemned the move and accused each other of trying to manipulate the vote.
President Muhammadu Buhari, of the APC party, urged calm and appealed to Nigerians to "refrain from civil disorder and remain peaceful, patriotic and united to ensure that no force or conspiracy derail our democratic development".
His main rival Atiku Abubakar has called for calm over the next seven days saying: "I'm appealing to Nigerians to please come out and vote and I'm asking them to be patient about it."
Voters have reacted with a mixture of anger, frustration and resignation.
Responses to Inec's tweet about the postponement were brutal with one man calling it "the height of incompetence".
Many had made long journeys to vote. In the northern town of Daura, Musa Abubakar, who had travelled 550km (342 miles) from Abuja to take part in the election, told the BBC that he "couldn't believe" what had happened.
Hajiya Sa'adatu said she was "greatly disappointed" to learn of the delay when she came out to cast her vote in the northern city of Kano.
But others have been saying that the postponement should mean that everything goes smoothly next week.
Michael Momodu, in Delta state south-east Nigeria, said that he still believed that "the right things will take place" and that "God will deliver".
Why have elections been postponed?
Election chief Mahmood Yakubu said the decision was made following a "careful review" of the election "operational plan", adding that there was a "determination to conduct free, fair and credible elections".
He said the delay was necessary to give the commission time to address vital issues and "maintain the quality of our elections", but did not provide further details.
In the past two weeks several Inec offices have been set alight, with thousands of electronic smart card readers and voter cards destroyed.
There have also been claims of shortages of election material in some of the country's 36 states.