Deadly clashes broke out in Bangui, the capital of the Central African Republic (CAR), after a Muslim motorbike taxi driver was killed.
About 20 people have been killed with at least 100 others wounded in the violence.
UN peacekeepers took up positions as the fighting spread from the city's only Muslim neighbourhood to an area where many Christians reside.
CAR has experienced religious and ethnic unrest for more than two years.
The BBC's Max Allaroum in Bangui says Saturday's violence is some of the worst the city has experienced this year - with the largest death toll
The UN Office for the Co-ordination of Humanitarian Affairs tweeted that residents were fleeing the clashes, heading to displacement camps in the city.
Amadou Rufai, a mayor in the Muslim neighbourhood, told BBC Afrique that militia from the mainly Christian "anti-balaka" militia got involved in the violence - and he feared this would mean the violence could continue.
Violence escalated in CAR in March 2013 when mainly Muslim Seleka rebels seized power in the majority Christian country.
Christian militias then formed and during that time about 25% of the population fled their homes.