At least 35 people have died in clashes between traders and militiamen in the Central African Republic capital, Bangui, the local Red Cross says.
Dozens more have been injured and about 50 shops burned down.
The fighting took place in the mainly Muslim PK5 district of Bangui, an area often troubled by violence.
The clashes started when shopkeepers took up arms against a local militia which has been trying to force them to pay taxes.
Ongoing violence since a rebel group seized power in 2013 has forced more than a quarter of the CAR’s 4.7 million people to flee their homes.
The UN mission in CAR, known as Minusca, has sent a rapid reaction force to the area to try and control the fighting.
It has not said how many people died but said there were “dozens of victims”.
The UN mission also urged both sides to “show restraint to avoid a new escalation of violence, with disastrous consequences for the civilian population”.
Following years of unrest and bad governance, the CAR is one of the world’s poorest countries.
Two-thirds of the population needs humanitarian aid, according to the UN.