Thousands of people have fled their homes in northern Nigeria after riots prompted by the election of incumbent President Goodluck Jonathan.
The Red Cross told the BBC some 16,000 has been displaced in six states across the north where some residents slept in police stations for safety.
Mr Jonathan appealed for an end to the violence and imposed a 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 disassociate 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 a 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'
Red Cross volunteers have treated 368 people injured in Monday's post-election clashes, some of whom have been taken to hospital.
"There were deaths, but we concentrated on the injured," Umar Marigar, the Red Cross's national disaster co-ordinator, told the BBC.
He added that the number of people displaced was likely to rise.
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, shots heard overnight continued into Tuesday morning.
In the north and south of the city, large numbers of youths have set up barricades and are clashing with the military, the BBC's Abdullahi Kaura Abubakar reports.
In Kano, the largest city in the north, police and soldiers are patrolling the streets and manning checkpoints.
But the BBC's Mansur Liman says the 24-hour curfew has been relaxed from dusk to dawn and people have started venturing out from their homes.
There were residents who fled their homes to seek safety at police barracks overnight - others sought refuge at hotels where they told the BBC about their experiences.
"Friends lost homes; 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 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.
Mannir Dan Ali, editor of Nigeria's Daily Trust newspaper, said the scale of the violence was a surprise, but not the tension in the north. The region suffers from high levels of poverty and unrest in places like the north-east where the radical Boko Haram Muslim sect is battling security forces.
"All this combined to produce a situation that was almost like a tinderbox," he told the BBC's World Today programme.
"Then when the results didn't seem to be going the way that most people voted, especially in their locality, there was this near spontaneous reaction that spread so fast."
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.
To win at the first round, a candidate not only needs the majority of votes cast, but at least 25% of the vote in two-thirds of Nigeria's 36 states. Goodluck Jonathan, of the PDP, reached that threshold in 31 states; runner-up Muhammadu Buhari of the CPC only did so in 16 states.
Nigeria's 160 million people are divided between numerous ethno-linguistic groups and also along religious lines. Broadly, the Hausa-Fulani people based in the north are mostly Muslims. The Yorubas of the south-west are divided between Muslims and Christians, while the Igbos of the south-east and neighbouring groups are mostly Christian or animist. The Middle Belt is home to hundreds of groups with different beliefs, and around Jos there are frequent clashes between Hausa-speaking Muslims and Christian members of the Berom community.
Despite its vast resources, Nigeria ranks among the most unequal countries in the world, according to the UN. The poverty in the north is in stark contrast to the more developed southern states. While in the oil-rich south-east, the residents of Delta and Akwa Ibom complain that all the wealth they generate flows up the pipeline to Abuja and Lagos.
Southern residents tend to have better access to healthcare, as reflected by the greater uptake of vaccines for polio, tuberculosis, tetanus and diphtheria. Some northern groups have in the past boycotted immunisation programmes, saying they are a Western plot to make Muslim women infertile. This led to a recurrence of polio, but the vaccinations have now resumed.
Female literacy is seen as the key to raising living standards for the next generation. For example, a newborn child is far likelier to survive if its mother is well-educated. In Nigeria we see a stark contrast between the mainly Muslim north and the Christian and animist south. In some northern states less than 5% of women can read and write, whereas in some Igbo areas more than 90% are literate.
Nigeria is Africa's biggest oil producer and among the biggest in the world but most of its people subsist on less than $2 a day. The oil is produced in the south-east and some militant groups there want to keep a greater share of the wealth which comes from under their feet. Attacks by militants on oil installations led to a sharp fall in Nigeria's output during the last decade. But in 2010, a government amnesty led thousands of fighters to lay down their weapons.