At least 80 people are believed to have been killed in fierce fighting on Thursday in the capital of the Central African Republic (CAR), Bangui.
The city was reportedly attacked by supporters of Francois Bozize, who was ousted as president by rebels in March, plunging the country into chaos.
The UN Security Council has just voted to allow French troops to join an African peacekeeping force in CAR.
About 10% of CAR’s 4.6 million people have fled their homes, the UN says.
French Foreign Minister Laurent Fabius told the BFM-TV news channel that the French deployment would total around 1,200, with 600 troops already in place.
About 250 French troops were sent out onto the streets of Bangui when the violence began on Thursday.
The Security Council resolution also imposed an arms embargo and asked the UN to prepare for a possible peacekeeping mission of its own.
Law and order broke down there after Mr Bozize was ousted by Michel Djotodia, who installed himself as the first Muslim leader in the Christian-majority country.
Mr Djotodia has officially disbanded his Seleka rebel group and integrated some of them into the army but correspondents say it is often not clear who is in charge, even in the capital.
An existing peacekeeping force about 2,500 strong, the International Support Mission in Central Africa (Misca), is already in place.
It is now under a 12-month UN mandate and will rise to some 3,600 African soldiers, in addition to the 1,200 from French forces.