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.
In a press conference on Saturday, the chairman of the Independent National Electoral Commission (Inec), Mahmood Yakubu, said the decision to delay had "nothing to do with political influence".
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."
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