Thousands of people have fled their homes in northern Nigeria after riots prompted by the election of incumbent President Goodluck Jonathan.
In some towns residents slept in police stations for safety. The Red Cross says there have been significant numbers of casualties.
Mr Jonathan appealed for an end to the violence and imposed a 24-hour curfew.
His main rival Gen Muhammadu Buhari, a northerner, told the BBC the violence was sad, unwarranted and criminal.
Some of the rioters have been alleging ballot-rigging, but the former military leader said he wanted to disassociated himself and his party from the clashes.
"In the last 24 hours there has been a spate of violence in the country this has included the burning of churches and is sad, unfortunate and totally unwarranted development," he said in a statement.
"I must emphasise that what is happening is not ethnic, religious or regional."
Mr Jonathan, a southerner, was declared winner of Saturday's presidential poll, with the electoral commission saying he received about 57% of the vote with 22.5 million votes to General Buhari's 12.2 million votes.
International observers have said the election was reasonably free and fair.
'I just saw people running'
In Kano, the largest city in the north, police and soldiers are patrolling the streets and manning checkpoints, the BBC's Mansur Liman reports.
Some people fled their homes to seek safety at police barracks overnight - other sought refuge at hotels where they told the BBC about their experiences.
"Friends lost home, I saw people who were killed," one man at a hotel in Kano told the BBC.
"I was at my place of work and I just saw people running, houses burnt," a woman added.
Another Kano resident described how young boys had entered her residential area threatened them, demanded money and grabbed mobile phones.
On Monday, homes displaying posters of Mr Jonathan were set on fire, and gangs of young men roamed Kano's streets shouting "Only Buhari!".
In Kaduna, where local TV stations reported that the house of Mr Jonathan's running mate, Vice-President Namadi Sambo, was set on fire in Monday's trouble, some shots were heard overnight.
But the BBC's Abdullahi Kaura Abubakar said there was an eerie calm in the city on Tuesday morning - with all businesses closed - as people observed the 24-hour curfew.
Some people are complaining about being unable to go and get food, as people tend to buy by the day, he says.
In the central city of Jos, where there was rioting in the Gangare area to the north of the city, some residents also slept overnight in police stations fearing further unrest.
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, whom he had served as vice-president.
He staked his reputation on the election, repeatedly promising it would be free and fair.
"I have received with great sadness the news of sporadic unrest in some parts of the country which are not unconnected with last Saturday's elections," he said after he was declared winner on Monday.
"I appeal to those involved to stop this unnecessary and avoidable conduct, more so at this point in time when a lot of sacrifice has been made by all the citizens of this great country in ensuring the conduct of free and fair elections.
"I call on all our political leaders, especially the contestants, to appeal to their supporters to stop further violence in the interest of stability, peace and well-being of this great country. No-one's political ambition is worth the blood of any Nigerian.".
