|Violence erupted in the north after Goodluck Jonathan was declared winner of the presidential election [Reuters]
Nigerians are set to vote in the nation's fiercely contested state governorship elections.
Tuesday's vote follows legislative and presidential elections earlier this month, in which hundreds have been killed and at least 40,000 people forced to flee their homes.
Election officials postponed the governors' races in the two northern states hardest hit by post-election violence but vowed to press ahead with voting in 36 other states.
Violence erupted in the north last week after Goodluck Jonathan, a southern Christian, was declared winner by a wide margin in the April 16 presidential election.
Supporters of his northern opponent Muhammadu Buhari rejected the results and took to the streets. Hundreds of people died in the ensuing violence and churches, mosques and homes were set ablaze.
Those displaced are sheltering in army barracks, where they are being looked after by aid agencies.
Some suggested this week's voting for state governors and state assemblies be postponed to allow tempers to cool, but Jonathan said the polls would go ahead despite the violence.
"This was a public declaration that he won't let anything stand in the way of completing this election cycle," Patrick Mmeme, a public policy analyst and writer, told the Reuters news agency.
State polls have in the past led to unrest in the Niger Delta, the southern heartland of Africa's biggest oil and gas industry, where politicians armed thugs to intimidate voters.
There has already been violence in some parts of the region, including rioting in the state of Akwa Ibom and attacks on rallies in Bayelsa, and the security forces are on high alert.
But this year. it is the north which provides the biggest security headache. In two northern states, Kaduna and Bauchi, where some of the worst violence took place last week, voting will be delayed until Thursday.
"The north has become a killing field as a consequence of growing social, ethnic and religious intolerance," Abdullahi Adamu, a former state governor elected senator for Nasarawa West in the region, told the Daily Trust newspaper.
"Violence has drawn the north back several decades. We have murdered our brothers and sisters in cold blood, destroyed our property and even infrastructure and places of worship for no just cause," Adamu, a prominent member of Jonathan's party, said.
Some commentators have expressed shock that rioters attacked the palaces of emirs, traditional Muslim rulers, in a part of the country where their authority was once respected.
Properties of northern politicians and businessmen known to have backed Jonathan, who became president when his predecessor died, are also reported to have been attacked.
Nigeria, Africa's most populous country, with more than 150 million people, is split almost equally between a mainly Muslim north and a majority Christian south, although large minority groups live in both regions.
The series of elections began with a parliamentary vote which was delayed by administrative chaos, followed by the presidential ballot. This week's polls will again be fiercely contested by all five main parties.
Much is at stake in the finale as state governors control big budgets in the oil-producing country, are closer to the people, and influence policy at state and federal level.
A successful conclusion of the elections, judged so far to have been the most credible since a return to democracy in 1999, could boost Nigeria's world standing and attract investment.