Nigeria's president and the leading opposition candidate have urged people to remain calm after a national election scheduled for today was postponed by a week just five hours before polls were due to open.

However, the opposition candidate, former vice president Atiku Abubakar, later accused President Muhammadu Buhari of instigating the delay to "disenfranchise" the electorate.

Early this morning, just as Nigerians began heading out to polling stations, Mahmood Yakubu, the chairman of the electoral commission said it was no longer feasible to hold free and fair elections due to logistical problems.

The vote will now be held on 23 February, the chairman said.

But the delay still threw the country, where some 84 million people have registered to vote, into renewed political uncertainty.

Past elections in Nigeria have been marred by violence, intimidation and ballot-rigging, and the postponement raised the possibility of unrest.

Mr Yakubu later said the postponement was caused by delays in transporting electoral materials.

"Our decision was entirely taken by the commission. It has nothing to do with security, nothing to do with political influence, nothing to do with availability of resources," Mr Yakubu, chairman of the Independent Electoral Commission (INEC), told a gathering of foreign election observers and reporters.

He added that some sensitive election materials had been distributed but all had been retrieved and returned to the central bank and an audit would now be conducted.

In Nigeria the central bank stores election materials for safe keeping.

Mr Buhari, in power since 2015, faces a tight election contest against Mr Atiku of the People's Democratic Party (PDP).

Nigeria is Africa's largest economy and its top oil producer.

PDP chairman Uche Secondus said the postponement was "dangerous to our democracy".

He called it part of an attempt by Mr Buhari to "cling on to power even when it's obvious to him that Nigerians want him out".

Mr Atiku and Mr Buhari both sought to avert any violent protests or confrontation.

Mr Buhari, who was Nigeria's military ruler in the early 1980s, said in a statement he was "deeply disappointed" and urged Nigerians to "refrain from all civil disorder and remain peaceful, patriotic and united".

Mr Atiku told reporters at his residence in the northeastern city of Yola that he was shocked by the postponement but urged voters to be patient.

He later said Mr Buhari's administration was behind the delayed elections, without offering any evidence.

"By instigating this postponement, the Buhari administration hopes to disenfranchise the Nigerian electorate in order to ensure that turnout is low on the rescheduled date," he said on Twitter.

Mr Buhari's spokesman Garba Shehu declined to comment.

Presidential elections in 2011 and 2015 were also delayed over logistics and security issues.