Calm has returned to most areas but many people have fled fearing more violence
Nigerian President Goodluck Jonathan has said the violence following his election is a "sad reminder" of events that plunged Nigeria into civil war.
He said Nigeria was still struggling to come to terms with the suffering of the 1967 conflict when the south-east tried to establish the state of Biafra.
Tens of thousands of people have fled the recent post-poll unrest.
The president said the violence was intended to frustrate remaining elections, but they would go ahead.
Riots broke out in the north on Monday after Mr Jonathan, a southerner, emerged as the winner of the presidential poll.
Muhammadu Buhari, who is popular in the north, denies instigating the "sad, unfortunate and totally unwarranted" events.
Nigeria is divided by rivalry between the predominantly Muslim north and the mainly Christian south, which also have cultural, ethnic and linguistic differences - so much so that the presidency has often alternated between people who come from each of the two halves of the country, in an attempt to keep the peace. ["often" alternated? How "often" has a Christian been President, in the half-century since independence? How "often" in the last 40 years, since the Biafra War?]
The polls for Nigeria's 36 powerful state governors are due to take place next Tuesday.
'Enough is enough'
In an address to the nation, President Jonathan said the "horrific acts" of the last few days had been shocking.
"They killed and maimed innocent citizens. They set ablaze business premises, private homes and even places of worship," he said.
"If anything at all, these acts of mayhem are sad reminders of the events which plunged our country into 30 months of an unfortunate civil war," he said referring to the Biafran war in which more than one million people died.
"As a nation we are yet to come to terms with the level of human suffering, destruction and displacement, including that of our children to far-away countries, occasioned by those dark days.
"Enough is enough," he said.
The BBC's Abdullahi Kaura Abubakar in Kaduna, the state which has witnessed the worst of the violence, says Kaduna city is now calm.
Security has been stepped up nationwide
But it is difficult to confirm what is happening in the south of the state where there have been reports of continuing trouble.
Kaduna's police say 32 people have died in the clashes - our reporter says the casualty figure may rise as Muslims tend to bury their dead quickly. [yes, but almost all of the victims in Kaduna were Christians]
He went to one hospital in the city and saw 25 charred corpses on a mortuary floor and was told there were another 25 bodies in the mortuary fridge but he had to leave without checking because of the stench.
On Wednesday, the Red Cross put the figure of those fleeing the violence at 48,000.
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Goodluck Jonathan, People's Democratic Party: 22.5 million votes (59.6%)
Muhammadu Buhari, Congress for Progressive Change: 12.2 million votes (32.3%)
Nuhu Ribadu, Action Congress of Nigeria: 2.08 million votes (5.5%)
Ibrahim Shekarau, All Nigeria Peoples Party: 911,455 votes (2.4%)
Figures: Independent National Electoral Commission
During his speech, the president said that security has been reinforced nationwide to quell any further unrest.
He added that there was no grievance that the law courts could not address.
Gen Buhari has said that his party will challenge some of the results - he maintains the election commission's computers were programmed to disadvantage his party in some parts of Nigeria.
But he urged his supporters to refrain from attacks, saying: "It is wrong for you to allow miscreants to infiltrate your ranks and perpetrate such dastardly acts as the mindless destruction of worship places.
"Needless to say, this act is worse than the rigging of the elections."
International observers have said the election was reasonably free and fair.
Mr Jonathan, a Christian from the oil-producing Niger Delta, was appointed to the presidency last year upon the death of incumbent Umaru Yar'Adua, a northern Muslim whom he had served as vice-president.
He staked his reputation on the election, repeatedly promising it would be free and fair.