* Some local forces ignore order to quit capital's streets
* Body count rises over 400 after three days of violence
* French force to reach 1,600 soldiers on Saturday
By Emmanuel Braun
BANGUI, Dec 7 (Reuters) - French soldiers were cheered by residents on Saturday as they began patrolling densely populated neighbourhoods of Central African Republic's capital, Bangui, which has been rocked by waves of killings between Muslim and Christian communities.
Attacks on civilians in Bangui continued for a third day, aid workers said. Groups of heavily armed gunmen sped around its dusty streets after the expiry of a 3 p.m. (1400 GMT) deadline for all forces, except foreign peacekeepers and the presidential guard, to return to barracks.
France is deploying 1,600 troops to its former colony, where at least 400 people have died in three days of violence between the Seleka rebel group that seized power in March and Christian self-defence militias, which has spilled over into religious violence in the capital and beyond.
Peter Bouckaert, Human Rights Watch's emergency director, said Seleka fighters burned houses in Bossangoa, about 300 km (190 miles) north of Bangui, after French jets flew overhead. They killed a woman, leaving her body next to her crying baby, he said.
Central African Republic has been gripped by chaos since Seleka toppled President Francois Bozize and embarked on months of looting, rapes and killings. Seleka's leader, Michel Djotodia, installed as CAR's interim president, has lost control of his loose band of fighters.
The U.N. Security Council authorised France on Thursday to use deadly force to help African peacekeepers struggling to restore order. Paris sprang into action after an attack on Bangui the same day by Christian militias and gunmen loyal to Bozize that ignited the worst violence in a year of crisis.
French helicopters and warplanes flew low over the dilapidated riverside capital on Saturday. Residents in the Combattants neighbourhood cheered French soldiers as they patrolled on foot down narrow streets.
"If they can get into the neighbourhoods, we might start seeing a reduction in these crimes," said Amy Martin, head of the U.N. Officer for the Coordination of Humanitarian Affairs, OCHA. "The level of atrocities and the lack of humanity, the senseless killing, defies imagination."
French reinforcements crossed into western Central African Republic from Cameroon on Saturday, while troops also moved northward from the capital in a bid to quickly pacify the sparsely populated interior, army spokesman Gilles Jarron said.
The troops reached Bossangoa shortly after nightfall.
The force's strength reached 1,200 troops by midday on Saturday and President Francois Hollande said 400 more were due by evening to cope with the unexpected scale of the violence.
"It's a number that will remain as long as necessary for this mission," Hollande told a news conference in Paris.
Hollande's office said the African Union force France is supporting would be increased to 6,000 men from 3,500.
"The country for the first time in its history is heading towards a religious war. We have to be very careful," Chadian President Idriss Deby, whose country has provided hundreds of troops to the mission, said after a meeting on the crisis.
About 10,000 Bangui residents fled to the airport, where French forces are based, in search of safety. Thousands more gathered at churches in town.
"Out-of-control Seleka fighters have been shooting all over the neighbourhoods so people cannot sleep," Topo Oubanguin, one of the displaced at the St. Jean de Galabadjia church, said.
Even if French forces manage to improve security, Hollande indicated Djotodia may be a spent political force.
"I don't want to point fingers, but we cannot keep in place a president who was not able to do anything, or even worse, who let things happen," he said on the France 24 TV channel.
Guy-Simplice Kodegue, a spokesman for the Bangui government, rejected Hollande's statement, saying Djotodia would stay in his post until the end of the transition in 2015.
Kodegue told France to stick to its task of supporting the African mission. "France's mandate is not to hurt a head of state on his own territory," he told Reuters.
PEACE 'WILL TAKE TIME'
France's dramatic intervention comes just months after it deployed 4,000 troops to oust al Qaeda-linked Islamist groups that had seized control of northern Mali. Paris has played down comparisons with the mission in Central African Republic, which is says is more humanitarian.
The landlocked nation has seen little but conflict and political instability since independence from France in 1960. Having intervened in previous conflicts, Paris hoped to avoid this one, but the level of slaughter forced its hand.
The Red Cross said that 394 bodies had been collected from Bangui's streets by Saturday evening. More were expected to be brought in on Sunday. Medical charity Medecins Sans Frontieres said it had treated 190 wounded in the Hopital Communautaire.
Pastor Antoine Mbao Bogo, president of the Red Cross in Central African Republic, said a number of attacks took place in Bangui on Saturday and some Seleka fighters continued to flout the deadline as night fell.
"We see the international forces, but there has not been any real impact on the ground. It will take time," he added.
Bangui residents, many in tears and covering their noses to avoid the smell of the dead, gathered at the city's main morgue late on Friday as the Red Cross brought in bodies.
"My brother was killed but I can't find his body here. I'll search the hospital," said a woman, giving her name as Pauline.
At least 30 people were killed in Bossangoa on Thursday but dozens more have died since, an aid worker said. (Additional reporting by Emmanuel Jarry, John Irish and Muriel Boselli in Paris, Joe Penney in Bamako; Writing and additional reporting by David Lewis; Editing by Sonya Hepinstall and Peter Cooney)