President Muhammadu Buhari on Friday urged Nigerians "to go out and vote", promising that there would be adequate security for Saturday's postponed election that pits him in a tight race with businessperson Atiku Abubakar.

The Independent National Electoral Commission (INEC) announced a week's delay to voting in the early hours of last Saturday, just as some of Nigeria's 72.8-million eligible voters were preparing to go to polling stations.

In a televised morning address on the eve of the vote, Buhari asked Nigerians to "cast aside doubt and have faith that INEC will rise to the occasion" on Saturday.

"Do not be afraid of rumours of violence and unrest. Our security agencies have worked diligently to ensure that adequate security measures are in place," he said.

The Boko Haram militant group and its offshoot, Islamic State in West Africa Province, have carried out deadly sporadic raids in the northeast's Borno state. Boko Haram has warned people not to vote.

Buhari's rival Atiku, a former vice president who is representing the main opposition People's Democratic Party (PDP), made a similar appeal to voters through his Twitter feed on Thursday night.

"This Saturday, a vote for PDP is a vote to get Nigeria working again. Come out, vote and #DefendYourVote," Atiku said in a tweet accompanied by a video of his rallies.

Buhari, a former military ruler who was later elected president in 2015, was criticised on Monday for saying that anyone trying to intimidate voters or interfere with the voting "will do it at the expense of his own life".

Atiku's party said Buhari's comments were a call for "jungle justice".

Members of Buhari's All Progressives Congress party and those of Atiku's PDP have accused each other of being behind the delay and colluding with the electoral commission.

Neither party has publicly provided evidence to back up their allegations.

The INEC's chairperson Mahmood Yakoob has insisted that the vote will go ahead on Saturday. He blamed logistical reasons for the postponement, and said there had been no external pressure.

Buhari faces a close contest against Atiku to lead a country that has Africa's largest economy and is its top oil producer, but is plagued by corruption and wide gaps between rich and poor.

The election has been fought over Buhari's handling of the economy amid fragile growth and growing unemployment as the country recovers from a recession in 2016.