Nigeria’s president and leading opposition candidate have appealed for calm after the shock move to delay elections for a week.
President Muhammadu Buhari said he was “deeply disappointed” but urged people to refrain from “civil disorder”.
Main rival Atiku Abubakar called for patience but accused the administration of “anti-democratic acts”.
Election officials cited “logistical” reasons for the 11th-hour delay in presidential and parliamentary polls.
No full details of why the overnight decision was made have yet been announced.
How have the political parties reacted?
The two main groups, the ruling All Progressives Congress (APC) and the People’s Democratic Party (PDP), both condemned the move and accused each other of trying to manipulate the vote.
President Buhari, of the APC party, urged 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, 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.”
In a tweet he said the administration was trying to disenfranchise the electorate.
The chairman of the PDP, Uche Secondu, said the delay was “dangerous to our democracy”, accusing Mr Buhari of trying to “cling on to power even when it’s obvious to him that Nigerians want him out”.
And the voters?
In many cases they reacted with disappointment, frustration and anger.
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.
“We spent all night without sleeping hoping to vote today,” Auwolu Usman, a voter in Maiduguri, told Reuters.
Oyi Adamezie, in the city of Warri, told Agence France-Presse: “I see this postponement of the election as a ploy to rig.”
Yusuf Ibrahim, in Abuja, told Associated Press: “I came all the way from my home to cast my vote this morning… I am not happy, I’m very, very angry.”
Election officials bore the brunt of the anger:
So why was voting postponed?
Not a great deal has been revealed.
Chairman of the Independent National Electoral Commission (Inec), Mahmood Yakubu, said: “Proceeding with the election as scheduled is no longer feasible.”
He 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”.
In the past two weeks several Inec offices have been set alight, with thousands of electronic smart card readers and voter cards destroyed.
Election officials told Western diplomats one big problem was getting ballot papers and results sheets to some of the 36 states.
Nigeria has also seen violence in the run-up to the elections and on Saturday 11 people were killed in an attack by Boko Haram militants south of Maiduguri.
‘Sour taste in voters’ mouths’
By Damian Zane, BBC News, in Lagos
At first it seemed like another bout of fake news, but disbelief was replaced by outrage and disappointment. The last elections, in 2015, were also postponed, but the announcement coming five hours before the vote is a new record.
Those Nigerians who had intended to vote would have planned their day around it. The whole country was essentially going to shut down as travel was restricted, so the only options were to stay at home or go to the polling station.
Many have also already travelled to the places where they were registered to vote. They will be left wondering what they will do for the next week.
This will have left a sour taste in the mouths of the electorate, which is already cynical about the political process.
What happens next?
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 contest between the two main presidential contenders is expected to be tight.
Presidential elections in 2011 and 2015 were also delayed.
How important is this poll?
The future of Africa’s most populous nation and largest economy is at stake.
Whoever wins will have to address power shortages, corruption, security problems and a sluggish economy.
There are 73 registered candidates in the presidential election, but campaigning has been dominated by President Buhari, 76, and Atiku Abubakar, 72.
Mr Buhari says he has built a strong foundation for prosperity, but his rival says Nigeria is not functioning.
Both men are from the mainly Muslim north of the country. While both are in their 70s, more than half of Nigeria’s 84 million registered voters are under 35.