* France says it will use force if necessary to get guns off streets
* French troops trade fire with ex-rebels
* Mobs attack men accused of being disarmed fighters
* Life resumes in other parts of city as scale of the killing emerges
By Emmanuel Braun
BANGUI, Dec 9 (Reuters) - French troops in Central African Republic clashed with gunmen in the capital Bangui on Monday as they searched for weapons in an operation to disarm rival Muslim and Christian fighters responsible for hundreds of killings since last week.
Shooting erupted near the airport in the morning after gunmen refused to hand over their weapons, and French forces later came under attack by former rebels in the city centre.
France said it was prepared to use force if fighters rejected calls to disarm or return to barracks.
France boosted its military presence in its former colony to 1,600 troops over the weekend as waves of religious violence swept across the country. At least 459 people have been killed in Bangui alone since Thursday, according to Red Cross officials.
"This is not an easy job, but our soldiers are well prepared," French Foreign Minister Laurent Fabius told France Inter radio.
The order to disarm had been broadcast on local radio in Bangui and "if that is not enough, force is going to be employed."
Central African Republic has spiralled into chaos since mainly Muslim Seleka rebels seized power in March and embarked on months of looting, raping and killing. Seleka's leader, Michel Djotodia, installed as the country's interim president, has lost control of his loose band of fighters.
EXCHANGES OF FIRE
Christian militias and gunmen loyal to ousted president Francois Bozize attacked Bangui on Thursday, the same day the U.N. Security Council authorised France to use lethal force to help African peacekeepers already struggling to restore order.
In an early test of France's resolve, its troops traded fire with gunmen near the airport Monday morning. A French army spokesman in Paris called the incident "insignificant".
"Many armed elements who held positions in Bangui have left their positions to go back to their barracks," Colonel Gilles Jaron said.
However, French troops again came under attack later in the day in the PK 5 neighbourhood from suspected Seleka fighters.
"It was an attempt to intimidate. We responded with 20mm cannon, then sent in a platoonto carry out clean-up operations," Captain Guillaume Fresse, spokesman for the French force in Bangui, told Reuters.
It was not immediately known whether there had been casualties in the two incidents.
As French forces manned checkpoints and patrolled the streets, pockets of crowd violence erupted in several districts.
In the Castor neighbourhood, a Reuters reporter watched as a crowd attacked a man they accused of being a disarmed Seleka fighter after French soldiers removed weapons from a house there, then left.
At a mosque in the PK 5 neighbourhood, the resident imam showed journalists the bodies of two men who he said had been beaten to death by Christians.
The arrival of French troops has been broadly welcomed in a city still struggling to emerge from a period that saw fighters, both Christian and Muslim, go door-to-door killing civilians as they cowered inside.
As French warplanes and helicopters flew low overhead and despite the violences, Bangui residents reappeared on the streets and some shops and market stalls reopened for the first time since last week.
"Yesterday we couldn't even come here to cross this road because the Seleka came and set up a base here," said a woman who gave her name only as Armelle. "Thank God the French came. If there's peace, things will get better."
However, the United Nations said it had counted some 72,000 people displaced by the violence currently staying in various sites around the city, including at the airport, where French troops and African peacekeepers have their base.
"There are still conflicts in some neighbourhoods. There's still killing," said Amy Martin, head of the U.N. aid agency OCHA in Bangui. "For now, we don't have the feeling that people are ready to go home.
Meanwhile information from elsewhere in the country, cut off from the capital since last week, began to trickle in.
A humanitarian worker in the town of Bossangoa said the number of dead there from several days of violence between Seleka and Christian "anti-balaka" militias formed in response to the violence had risen to 38.
In Bozoum, in the northwest, U.N. officials received reports of dozens of dead, and there was also violence in the nearby town of Bocaranga.
Humanitarian agencies and rights groups said the figures only reflected bodies that had been officially counted, and that the final death toll was likely to be significantly higher.
"We've spoken to a lot of people who have just buried their relatives in the back yard because they couldn't get out or didn't see the point of calling the Red Cross," said Joanna Mariner, a crisis expert with Amnesty International in Bangui. (Additional reporting by Marine Pennetier, Joe Bavier and Bate Felix, Writing by Joe Bavier,; Editing by Angus MacSwan)