A major French military intervention in the Central African Republic (CAR) is expected within days after the UN security council authorised French and African troops Thursdayto use force to protect civilians as the world races to prevent a sectarian war.

At least 105 people were killed during the heaviest clashes for months between rival militias in the capital, Bangui, according to Reuters. Family members crowded into a mosque in the capital where 53 bodies were laid out, most of which appeared to have been clubbed or hacked to death.

The CAR has been heading towards sectarian warfare since March when mainly Muslim rebels, aided by mercenaries from Chad and Sudan, seized power in a coup in this predominantly Christian nation.

Crackles of gunfire erupted at around 6am in Bangui on Thursday and could still be heard sporadically nearly three hours later close to the city's international airport. There were other reports of arms fire from suburbs north and east of the city. Even the president's and prime minister's homes were looted.

Sylvain Groulx of Médecins Sans Frontières, said he had seen more than 20 bodies lying on a street and a further eight in a hospital morgue. His staff were treating up to 70 casualties "and that tally is going up". He said: "I'm worried what kind of reprisals there will be later. Nobody is moving and it's a real ghost town."

Former rebels known as the Seleka, accused of numerous atrocities and recruiting child soldiers, clashed with militias called the "anti-balaka" – the name adopted by mainly Christian vigilante groups – amid growing tit-for-tat violence along sectarian lines.

Amnesty International said many of those involved in the latest violence are believed to be child soldiers, some reportedly armed with machetes, iron bars and other basic weaponry. Christian Mukosa, a CAR researcher for Amnesty, was among guests holed up at the Ledger hotel in Bangui. "Every minute we are receiving horrifying information about what is going on in the city," he said. "People who are wounded are being taken from hospital on suspicion of being anti-balaka and executed. We are also receiving information about home searches by the Seleka. Everyone suspected of being anti-balaka is taken away and we don't know what happens to them."

With the city in virtual lockdown, people were unable to get urgent medical treatment, Mukosa added. "People are dying because no one can go in or out. Children can't get to doctors and nurses. I spoke to one woman who was trying to reach a doctor with a boy who had been shot but the boy died in her arms."

By afternoon, the dilapidated streets were empty of all but military vehicles and the four-wheel-drive pickup trucks favoured by the Seleka.

Michel Djotodia, who installed himself as the CAR's first Muslim president but now struggles to control the Seleka, accused supporters of ousted president François Bozizé of mounting the attack and ordered an overnight curfew. General Arda Hakouma, Djotodia's head of security, said "anti-balaka" forces were also involved. "There are many of them," he told Reuters. "Some of them are well armed with rifles and rocket launchers."

Prime minister Nicolas Tiangaye confirmed his house had been looted, describing the attackers as a group of Seleka who arrived in three pickup trucks. "It's true: my house was attacked and pillaged," he told AP, adding that his family was evacuated beforehand and was safe.

One Bangui resident said he saw a group of about 40 "anti-balaka" fighters in the Ngaragba district. Armed with AK-47 rifles and rocket launchers, they broke open the doors to the prison. One of the fighters told the resident: "Stay at home. Show us the houses of the Muslims."

Another resident, Wilfred Koyamba, told Reuters: "When Seleka entered, there were dead Christians. This time it could be worse. We need the French. The French have to come quickly."

There is currently a 2,500-strong regional peacekeeping force in the CAR. The African Union (AU) is due to take charge of that force this month and boost its size to 3,600 troops.

The UN security council resolution authorised the deployment of the AU force for one year, with the operation to be reviewed after six months. It also authorised "the French forces in the CAR, within the limits of their capacities and areas of deployment, and for a temporary period, to take all necessary measures" to support the African peacekeepers.

France, which already has about 400 troops based at the airport in Bangui, is preparing to send at least 1,000 additional soldiers to its former colony. But some observers feel this will still not be enough to have a significant impact on a country bigger than France itself.

French president François Hollande issued a statement after the UN vote. "The situation in the Central African Republic has become alarming, and even terrifying. Massacres are being committed there at this very moment, even in hospitals," he said. "Each day women and children are attacked and millions of displaced people are seeking shelter."

He said it was France's "duty" to act.

"Given the urgency, I have decided to act immediately, that is to say as of this evening in coordination with the Africans and with the support of European partners. Already 600 French troops are there. This number will be doubled within a few days, if not a few hours."

He added: "A year ago, in Mali, France was called upon to combat a terrorist invasion. She did so. Today, in very different circumstances, France is expected in order to avoid an humanitarian catastrophe. She will be there.

"I have every confidence in our soldiers to carry out this operation. I know their sense of duty, and their great professionalism. This intervention will be rapid and is not expected to last long. I am sure of its success."

He said all the necessary information and explanation about the operation would be given to the French parliament next week.France's military command insists any intervention is a "security operation" and not a war. The aim is to secure the capital Bangui and the principal routes towards neighbouring Cameroon and Chad.

Gérard Araud, France's UN ambassador, told France 24: "This is a real humanitarian crisis. The CAR is collapsing and it could lead to mass atrocities, with each community trying to slaughter the other. What you have right now in the CAR, on both sides, are thugs who are raping and plundering. We need to restore law and order."

He said the operation would "probably be easier" than France's offensive in Mali against Islamist fundamentalists. "Here we are up against armed thugs, not an organised opposition."

The security council authorised French and African troops to use force to protect civilians, imposed an arms embargo on the country and asked the UN to prepare for a possible peacekeeping mission. In a unanimously adopted resolution, it also asked secretary-general Ban Ki-moon to establish an inquiry into human rights abuses in the landlocked nation of 4.6 million people.