Many people are being treated in hospital after the clashes
At least 200 people have been killed in violence between Christians and Muslims in the Nigerian city of Jos, says the monitoring group, Human Rights Watch.
Troops ordered by Nigeria's vice-president to help police restore order have arrived and are patrolling the streets, enforcing a 24-hour curfew.
The fighting, which broke out on Sunday, has prompted thousands of people to flee the city.
Houses, mosques and churches have been burnt down and many people arrested.
It is believed to be the first time Goodluck Jonathan has used executive powers since President Umaru Yar'Adua left Nigeria for hospital treatment in Saudi Arabia in November.
Lt Col Shekari Galadima, a spokesman for the 3rd Division of the Nigerian Army, told the BBC's Network Africa programme the streets were calm and the troops in control of the situation.
The area has seen several bouts of deadly violence in recent years.
At least 200 people were killed in an outbreak of fighting between Muslims and Christians in 2008, while some 1,000 died in a riot in 2001.
The current violence has forced at least 3,000 people from their homes.
On Tuesday the violence spread beyond the city boundaries to neighbouring areas.
Shehu Saulawa, BBC Hausa
Jos has long been a time-bomb waiting to explode.
The town is split into Christian and Muslim areas. The divisions have been perpetuated by Nigeria's system of classifying people as indigenes and settlers.
Hausa-speaking Muslims have been living in Jos for many decades but are still classified as settlers, meaning it is difficult for them to stand for election.
The two groups are also divided along party political lines with Christians mostly backing the ruling PDP, and Muslims generally supporting the opposition ANPP.
In Nigeria, political office means access to resources.
The death toll has not been verified independently and it is not known how many Christians have died.
Human Rights Watch say at least 200 have died in the latest outbreak of violence.
Balarabe Dawud, head of the Central Mosque in Jos, told AFP news agency he had counted 192 bodies since Sunday.
Muhammad Tanko Shittu, a mosque worker who was helping to prepare mass burials, told Reuters he had counted 149 bodies.
Jos is in Nigeria's volatile Middle Belt - between the mainly Muslim north and the south where the majority is Christian or follow traditional religions.
Correspondents say such clashes in Nigeria are often blamed on sectarianism.
However, poverty and access to resources such as land often lie at the root of the violence.
It is unclear what the trigger was for the latest bout of violence.
Plateau State spokesman Dan Manjang told Network Africa there were reports that it may have started after a football match.
But he said it would be surprising if football was the reason.
Reuters quoted residents as saying the violence started after an argument over the rebuilding of homes destroyed in the 2008 clashes.