The fighting in three mostly Christian villages in the west African country appears to be revenge attacks after violence in the central Jos region in January when most of the victims were Muslims, the said Red Cross said.
By Sunday afternoon at least 200 bodies including those of several children and a baby in a nappy were piled up in a local morgue. One young victim appeared to have been scalped, and others had severed hands and feet. One woman victim was stripped below the waist.
Yesterday afternoon, government adviser Dan Manjang put the death toll at 500. "We have been able to make 95 arrests but at the same time over 500 people have been killed in this heinous act," he said.
Mass burials will be arranged for the victims, another state official said.
About 15,000 people have fled their homes to escape the riots that started after the election of President Goodluck Jonathan on Saturday. By yesterday, the violence had spread to 14 states.
Mr Jonathan has appealed for calm and brought in a 24-hour curfew. And his main rival Gen Muhammadu Buhari said the violence was ‘sad, unwarranted and criminal.’ Some of the rioters have been alleged ballot-rigging, but the former military leader said he wanted to distance himself and his party from the clashes.
The region’s electoral commission said Goodluck Johnathan had received about 57per cent of the vote with 22.5 million votes to General Buhari's 12.2 million votes.
"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," former military leader Buhari said in a statement.
"I must emphasise that what is happening is not ethnic, religious or regional."
Police and soldiers are patrolling the streets and manning checkpoints in Kano, the largest city in the north.
Hundreds of people fled their homes to seek refuge at police stations overnight - other sought safety at hotels.
"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.
Nigeria has a history of rigged and violent elections but Saturday's vote was deemed by many Nigerians and observers to have been an improvement on the past.
"Election day showed a generally peaceful and orderly process," said chief European Union election observer Alojz Peterle. EU observers said 2007 elections were not credible.