|Goodluck Jonathan, centre, has repeatedly promised a free and fair election over the next three weekends [Reuters]
Nigeria has tightened security and closed its land borders as the country prepares for a landmark election period to pick new leaders and legislators.
Authorities announced a reshuffling of state police chiefs a day before Saturday's parliamentary polls.
Areas at risk of violence took precautions, with mosques in central Nigeria installing metal detectors.
Al Jazeera's Yvonne Ndege reports on women vying for votes in the upcoming elections
Voting to elect a new legislature, president and state governors will take place over three consecutive weekends.
Land borders closed at noon on Friday and were to reopen on Sunday morning, the interior minister said in a statement, while vehicles will be restricted on roads on polling day.
Nigeria is Africa's most populous nation and largest oil producer.
The national police chief announced the redeployment of commissioners, a move questioned by one vice presidential candidate running for an opposition party.
"The posting is to position the commissioners of police for effective performance of their duties during the forthcoming general elections," police said in a statement.
The election period is significant in Nigeria's history as a test of whether it can organise a credible ballot after a series of deeply flawed and violent polls.
Goodluck Jonathan, the incumbent Nigerian president, is the favourite in the April 9 presidential poll, having repeatedly promised free and fair elections.
Muhammadu Buhari, a former military ruler, is seen as his main challenger. Governorship and state assembly elections will take place on April 16.
Fresh electoral list
A recently-installed electoral commission headed by a respected academic has raised hopes that the vote will be better conducted this time.
"We're worried about this intrusion by people who are not saddled constitutionally with the responsibility for conducting the elections"
Fola Adeola, Nigerian vice presidential opposition candidate
The commission has done away with an old electoral list littered with false entries and created a new one by taking electronic prints of every potential voter before issuing them a card.
Fola Adeola, the vice presidential candidate for the opposition Action Congress of Nigeria party, told AFP he was "reasonably satisfied" with the work of the electoral commission.
But he said he had concerns about whether other institutions were interfering in the elections, questioning why soldiers were being deployed on election day and the timing of the redeployment of police chiefs.
"We're worried about this intrusion by people who are not saddled constitutionally with the responsibility for conducting the elections," he said.
There have been outbursts of violence in the run-up to the polls, including bomb blasts and attacks on political rallies.
In the volatile central city of Jos, which has been for years hit by clashes between Christian and Muslim ethnic groups, mosques installed metal detectors for Friday prayers.
On Thursday, two bombs were discovered and defused at a rally attended by thousands in support of Buhari in his home state of Katsina in northern Nigeria, his campaign said.