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.

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, 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 insecurity.

The Boko Haram militant insurgent 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.

Witnesses and security officials told Reuters an attack in Borno on a governor's convoy on route to an election rally on Feb. 12 was deadlier than the government had said.

More than 1,000 soldiers from neighbouring Chad, belonging to a multinational joint force comprised of troops from the region, crossed the border into Nigeria on Friday to help with the fight against Islamists, according to two Chadian military sources.

Police patrol vehicles were seen moving around Borno's capital, Maiduguri, on Friday. Electoral commission vehicles were also seen ferrying election material under heavy police guard.

Australian Associated Press