The army sent reinforcements to police a 24-hour curfew on the city of Jos, which lies at the crossroads of Nigeria's Muslim north and Christian south, after rival groups of youths burned homes, shops, mosques and churches.
Hundreds of bodies were brought to the town's main mosque in preparation for a mass burial.
"I counted 218 dead bodies at Masalaci Jummaa. There are many other bodies in the streets," said a Red Cross official who asked not to be named.
That death toll did not include hospital figures, victims already buried, or those taken to other places of worship, meaning the final count could be much higher, officials said.
Muslims and Christians generally live peacefully side by side in Nigeria, Africa's most populous nation.
But ethnic and religious tensions in the country's "middle belt" have simmered for years, rooted in resentment by indigenous minority groups, mostly Christian or animist, toward migrants and settlers from the Hausa-speaking Muslim north.
The governor of Plateau state, of which Jos is the capital, said in a statement that troops had orders to shoot on sight to enforce the curfew in neighborhoods hit by the violence.
About 7,000 people fled their homes and were sheltering in government buildings and religious centers, the Red Cross said.