Ballot papers have not yet arrived in Nigeria on the eve of the presidential elections, the election commission head has said.
Maurice Iwu said the start of polling would be delayed by two hours and would now begin at 1000 (0900 GMT).
Correspondents say it is hard to see how 60m ballot papers can be delivered across the vast country in time.
EU observers say they are concerned about the credibility of the vote after last weekend's flawed state elections.
It is understood the ballot papers are being shipped from South Africa.
Mr Iwu made the announcement during a news conference broadcasted live on television, when he was asked about reports that a truck full of ballot papers already marked in favour of the ruling party had been seized.
"They couldn't have been captured because the ballot papers are arriving as we speak," he said.
After decades of mismanagement, Nigeria has a very poor road network. It stretches from the deserts of the north to the swamps and creeks of the oil-rich Niger Delta.
The Independent National Election Commission (INEC) chairman said the 65m ballot papers had to be printed abroad because it was not possible to do this in Nigeria in just three days.
However, the names of the presidential candidates are not on the papers - just the symbol of their political party.
On Monday, Vice-President Atiku Abubakar obtained a Supreme Court ruling that his name should be added to the presidential contest.
After violence and fraud in last week's state elections, Mr Abubakar and other opposition candidates called for the presidential poll to be postponed.
He said he was only taking part in order to be able to challenge the results in court.
President Olusegun Obasanjo has admitted there were flaws in the state polls and urged election officials to prevent rigging in the presidential vote.
"The world is watching us and we cannot afford to disappoint ourselves, our friends and the world," the outgoing leader said.
The head of the European Union monitoring team has said there must be major improvements in the way the polls are conducted if they are to be credible.
Max van den Berg said there had been widespread fraud during last weekend's state elections.
He said more than a quarter of Nigeria's 36 states had registered false results.
During a national address, Mr Obasanjo said no election could be regarded as perfect, but said progress had been made since elections in 1959.
"There have been allegations of malpractices, of multiple voting, ballot box snatching, coalition manipulation, intimidation, threats and use of violence. All these must be roundly condemned no matter who engaged in them," he said.
"I appeal to our local and international observers to understand some of our limitations as a complex developing nation and not to exaggerate the negative and thereby throw out the baby with the bath water."
Meanwhile, Friday was declared a surprise public holiday, to allow people time to travel before the vote.
Many people would have turned up to work to find their offices closed, as the announcement came late on Thursday night.
Nigeria is Africa's most populous country and is one of the world's biggest oil producers. It is of key strategic interest to both the West and the growing economies of the East.
But despite the country's huge oil wealth, tens of millions live in poverty without basic amenities.