A Reuters witness and an aid worker said at least 105 people were killed in fierce fighting in Bangui between mainly Muslim former rebels now in charge of the country and a mix of local Christian militia and fighters loyal to ousted president Francois Bozize. Many were civilians.
Mindful of the 1994 genocide in Rwanda, when hundreds of thousands were killed as the world looked on, the United States and other Western powers have urged swift international action to prevent the anarchy in Central African Republic leading to atrocities against the civilian population.
Most of the fighting in Bangui had eased by midday and the streets were largely deserted, but the death toll mounted and there were reports of widespread abuses during the fighting.
"I have decided to act immediately, in other words, this evening," French President François Hollande told reporters, hours after a vote at the U.N. Security Council authorized French and African troops to use force to protect civilians.
An arms embargo was also imposed on the country and the Security Council asked the United Nations to prepare for a possible peacekeeping mission.
France has about 650 troops based at Bangui airport. Some 250 of these were deployed in town on Thursday to protect French interests and citizens. Hollande said the numbers of French troops present in the country would be doubled as early as this evening due to reinforcements from neighbouring states.
Hundreds of French troops had been pre-positioned in Cameroon, Gabon and Chad, pending the U.N. approval to help restore order in Central African Republic.
The former French colony has slipped into chaos since mainly Muslim rebels seized power in March, leading to tit-for-tat sectarian violence with the Christian majority. However, the violence on Thursday was the worst the capital has seen during this year's crisis.
"We've received numerous reports from very credible sources of extrajudicial executions," said Joanne Mariner, a crisis expert with Amnesty International who is in Bangui.
"This underscores the need for international troops to arrive and secure the city. The situation is quickly spiralling out of control," Mariner said.
Fifty-three bodies had been brought to a mosque in Bangui's PK5 neighbourhood. Most victims appeared to have been clubbed or hacked to death, a Reuters witness said.
Samuel Hanryon, who works for medical charity Médecins Sans Frontières at Bangui's Hopital Communautaire, said there were another 52 bodies at the morgue there.
African peacekeepers protecting hundreds of civilians in their base in Bossangoa about 300 km north of the capital also came under heavy fire from the former rebels on Thursday, witnesses said.
Central African Republic is rich in gold, diamonds and uranium but decades of instability and spillover from conflicts in its larger neighbours have kept it mired in crisis.
Michel Djotodia, leader of the Seleka former rebel alliance, is now the country's interim president but he has struggled to control his loose band of fighters, many of whom are gunmen from neighbouring Chad and Sudan.
Mainly Christian local defence groups, known as "anti-balaka", have sprung up in response to abuses committed in Bangui and other parts of the country by the former rebels.
Some 400,000 people, or 10% of the population, have been force from their homes.
Djotodia and Nicolas Tiangaye, his prime minister, accused Bozize loyalists of mounting the Bangui attack. General Arda Hakouma, Djotodia's head of security, said "anti-balaka" forces, armed with rifles, rocket launchers and machetes, were also involved.
MSF said it treated 90 wounded at Bangui's Hopital Communautaire hospital. A witness said African peacekeepers ignored pleas of civilians for them to protect the hospital from marauding fighters. Many staff had fled, he said.
A separate witness said he saw dozens of bodies lying in the abandoned Ouango market in the southeast of the city.
The clashes appeared to have started around the Boy Rabe neighbourhood, a stronghold of Bozize that has been repeatedly raided by Seleka forces amid reports arms had been distributed to civilians before the former president fell.
There were reports of arms being handed out to civilians in the mainly Muslim PK5 neighbourhood.
The government declared an overnight curfew and closed its border with Democratic Republic of Congo.
Some rights groups have called for a U.N. peacekeeping mission to be set up immediately but regional leaders want to see if a beefed-up African force supported by France can contain the violence.
Having previously intervened in CAR's conflicts, Paris initially sought to avoid this one. But the scale of the violence since the rebels swept south has forced France's hand.
"When Seleka entered, there were dead Christians. This time it could be worse ... We need the French. The French have to come quickly," Wilfred Koyamba, a Bangui resident told Reuters.
Another resident said he saw a group of about 40 heavily armed "anti-balaka" fighters in the Ngaragba neighbourhood break open the prison doors there. One of the fighters told the resident: "Stay at home. Show us the houses of the Muslims."
Some Seleka gunmen had stripped off uniforms to blend into the population, witnesses said.
A French diplomat said on Thursday that 850 Burundian troops would be sent over the weekend to support the African mission.