France takes 'immediate' military action amid CAR clashes
A French-led military operation to protect civilians in the Central African Republic is being launched "immediately", after scores died in fresh sectarian fighting on Thursday.
A contingent of 650 troops there will be "doubled within a few days, if not a few hours," President Hollande said.
Earlier the UN Security Council voted to allow French troops to join an African peacekeeping force in the CAR.
Violence there has raised fears of mass killings along sectarian lines.
"I have decided to act immediately, in other words, this evening," Francois Hollande said.
Mr Hollande said the French role would be different to the one mounted in Mali, where French troops hunted down Islamist militants in the desert.
This time they will be the "gendarme" - the thin blue line between order and chaos, says the BBC's Christian Fraser in Paris.
CAR Prime Minister Nicolas Tiangaye welcomed the move, and said the immediate priority was to re-establish security in the capital, Bangui.
However, he warned that "when it's necessary to plan securing the country's other regions, this number of troops will be insufficient".
Bangui was attacked on Thursday, reportedly by militias loyal to Francois Bozize, who was ousted as president by rebels in March, plunging the country into chaos.
One hundred people were confirmed killed, Amy Martin from the UN in Bangui told the BBC.
A government curfew came into effect at 18:00 local time (17:00 GMT) but sporadic gunfire could still be heard, Ms Martin said.
The BBC's Andrew Harding, in Bangui, reported seeing many people lying wounded in the halls of a hospital.
Our correspondent says violence in the CAR is increasingly pitting Christians against Muslims.
For now, it appears that forces linked to the Seleka alliance remain in control, our correspondent says, adding that the situation is fluid and unstable, and there are deep concerns about more inter-communal violence across the country.
An existing peacekeeping force about 2,500 strong, the International Support Mission in Central Africa (Misca), is already in place.
It is now working under a 12-month UN mandate and will rise to some 3,600 African soldiers, in addition to the 1,200-strong French force.
The UK military says it is discussing "limited logistical support" for the French mission but will not send combat troops.
Michel Djotodia, who ousted Mr Bozize, installed himself as the first Muslim leader in the Christian-majority country.
The mainly Muslim Seleka rebel coalition which brought him to power has been accused of atrocities against Christians.
Christian communities have now set up "anti-balaka" self-defence forces, most of them loyal to Mr Bozize.
"Balaka" means machete in the local Sango and Mandja languages.
The Selekas have been officially disbanded and some of them integrated into the army, but correspondents say it is often not clear who is in charge, even in the capital.