Thousands of people have fled their homes in northern Nigeria after riots prompted by the election of incumbent President Goodluck Jonathan.
(BBC) - 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.
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.