* Heavy and small arms fire ring out before dawn
* Former rebels say city attacked by militia, former regime loyalists
* Fighting comes hours before U.N. vote on French mission (Adds further details of fighting)
By Emmanuel Braun and Paul-Marin Ngoupana
BANGUI, Dec 5 (Reuters) - Rival milita forces clashed in Central African Republic's capital on Wednesday hours before the U.N. was expected to authorise a French mission to halt Muslim-Christian sectarian violence that threatens to escalate into widespread massacres.
Former rebels now in charge of the country said Bangui had come under attack from local militia and fighters loyal to ousted president Francois Bozize. A Reuters witness at one hospital said he saw least 23 dead and 64 wounded in shooting since dawn. Another eye witness saw dozens of dead elsewhere.
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 are lobbying for international action to prevent the anarchy in Central African Republic from leading to major atrocities against the civilian population.
Most of the fighting had eased by midday, though the streets were largely deserted and there were reports of mounting death tolls and widespread abuses during the fighting.
"We've received numerous reports from very credible sources of extrajudicial executions," said Joanne Mariner, a crisis expert with Amnesty International who is currently in Bangui.
"This underscores the need for international troops to arrive and secure the city. The situation is quickly spiralling out of control," Mariner added.
The U.N. Security Council is due to vote later on Thursday on dispatching hundreds of French reinforcements to restore order in the country, which has slipped into chaos since mainly Muslim rebels seized power in March, leading to tit-for-tat sectarian violence.
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 up-country by the former rebels.
Djotodia accused Bozize loyalists of mounting the attack and ordered an overnight curfew. General Arda Hakouma, his head of security, said "anti-balaka" forces were also involved.
"There are many of them. Some of them are well armed with rifles and rocket launchers. Others are dressed in civilian clothes with machetes," Hakouma said.
Helen van der Velden, head of mission for MSF-Holland, which has a team in Bangui's Hopital Communautaire hospital, would not comment on the dead seen at the hospital but said staff had reported "numerous bodies in the streets".
A witness at the hospital said soldiers from a regional peacekeeping force already deployed in the country brought in about a dozen wounded. They then left despite the pleas of civilians for them to stay and protect them from marauding fighters. The hospital's staff had fled.
A separate witness said he saw dozens of bodies in the 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.
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.
The U.N. vote on whether to increase the French deployment is due at 1500 GMT.
"The current situation is unacceptable," the United Nations, African Union, European Union and France said in a joint statement on Thursday. "This attack led to loss of human life and targeted assassinations."
"WE NEED THE FRENCH"
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.
France has about 650 troops based at Bangui's airport, which they are protecting. Some 250 of these were deployed in town on Thursday to protect French interests and citizens.
Hundreds of others are pre-positioned in Cameroon, Gabon and Tchad, pending U.N. approval for the larger force due to help the struggling African peacekeeping mission restore order.
France's Foreign Minister Laurent Fabius said he expected the resolution to be adopted unanimously later on Thursday and French troop numbers would reach 1,200 "relatively quickly".
"The objective is to end this humanitarian tragedy, re-establish security and help the democratic transition," Fabius told RMC radio on Thursday morning.
In a sign of the spreading violence, earlier this week, the United Nations said armed men killed at least 12 civilians and wounded 30, including children, northwest of Bangui. (Additional reporting by David Lewis in Dakar, Marion Douet and John Irish in Paris and Joe Bavier and Ange Aboa in Abidjan; Writing by David Lewis; Editing by Joe Bavier and Ralph Boulton)