Nigeria's president has admitted there were flaws in last week's state polls and urged election officials to prevent rigging in the presidential vote.
President Obasanjo said no election was perfect
"The world is watching us and we cannot afford to disappoint ourselves, our friends and the world," outgoing leader Olusegun Obasanjo said in a speech.
The two main opposition presidential candidates say they will contest Saturday's polls despite fraud fears.
Atiku Abubakar said he would take part only so he could challenge the result.
NIGERIAN ELECTION 21 APRIL
60m registered voters
120,000 ballot boxes
360 House of Representative seats to be elected
109 Senate seats to be elected
24 presidential candidates
Main contenders: Atiku Abubakar for the AC, 60 years oldMuhammadu Buhari, ANPP, 64Umaru Musa Yar'Adua, PDP, 55
Polls open at 0800 (local time) and close at 1500
To avoid a run-off, a candidate needs the highest number of votes overall and at least 25% of votes in 24 of the 36 states
"You have to participate to challenge the wrongdoings in the courts," Vice-President Abubakar, who is standing for the opposition Action Congress, told the BBC's Focus on Africa programme.
The All Nigeria People's Party's Muhammadu Buhari also confirmed he would take part.
He denied that the opposition's failure to field a single candidate would mean victory for the ruling People's Democratic Party.
"It hasn't handed it over. If that is your assumption, it is the wrong assumption," he said.
The opposition says the Independent National Election Commission (Inec) should be disbanded, saying it is biased in favour of the PDP.
Friday has been declared a surprise public holiday, to allow people time to travel before the vote on Saturday.
The BBC's Peter Okwoche in Port Harcourt says many people would have turned up to work to find their offices closed, as the announcement came late on Thursday night.
During his national address, Mr Obasanjo said no election could be regarded as perfect, but said progress had been made in Nigeria 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."
After last weekend's vote, Nigerian observers said results from a third of the states should be annulled and there is concern about the credibility of Saturday's polls.
The deputy head of the European Union observer mission has asked Inec to take urgent action.
"I fear that if they don't put some of these things in place, then there is a danger that there will be not only non-credible elections on Saturday but there won't be secure elections and that there could be resulting violence," Graham Elson said.
Nigeria is Africa's most populous country and is one of the world's biggest oil producers.
But despite the country's huge oil wealth, tens of millions live in poverty without basic amenities.
The BBC's Alex Last says Nigeria is of key strategic interest to both the West and the growing economies of the East.
He says it matters who wins and whether they are a credible leader.
The danger is a failed election simply moves Nigeria further down the road to instability, our reporter says.