There can be few more anguishing sights than seeing the body of a murdered 15-year-old boy being removed from his family home on an ordinary London street where he was raised by loving parents. The death of William Cox, who died in the bloodied, cradling arms of his 13-year-old sister, is a grim distress call from London's poisoned well of guns and gang culture.

The tributes from his friends dubbed him "Remer", his graffiti tag, and praised him as a "fallen soldier". But you look at his baby-faced features in newsprint, at the railroad hair-styling and the bling ear-ring, and staring back is an inner-city kid, a member of the army of lost boys.

"Billy" Cox was the third teenager to be killed in the last fortnight and one of five murder victims in a spasm of violence that has jolted Britain's capital out of its complacency on gun culture. Two of the youngsters have been murdered in their own homes and a third gunned down at an ice rink in front of hundreds of witnesses.

On Friday night the police were out, armed to the teeth, to prove that they have not lost control the streets of south London. That left a gap in the market in Hackney, in the city's east end, where a man in his 20s was fatally wounded. Politicians are calling for fathers to take responsibility for child rearing, the home secretary, John Reid, is ready to consider a change in the law. There is much hand-wringing over what can be done. On the streets the residents of Clapham and Peckham are just frustrated and broken-hearted.

People going about their daily business are aghast at the hormone-fuelled, murderous bedlam that has erupted in their midst. Most people living on what the media have dubbed the "mean streets" can't fathom the perverse nature of the violence that has been visited on them. But they do feel the ripples of insecurity the murders send through the fabric of their lives.

"It used to be that you told your children to come straight home after school and keep the door closed - they would be safe. Now, no one is so sure," says Elena Knight as she waits for a number 12 bus to whisk her the four miles northwards to Oxford Street and the safe, familiar iconography of what we regard as London.

South London, a chain of genteel, urban villages strung between great swathes of inner city deprivation is less visited and less well known. Millions of pounds have been spent on regeneration projects and on policing the murderers and drug dealers that blight the black community. But although the backdrop has changed, acts of wanton violence, gang or drug-related, are still played out on this grey stage. The difference now is that the players are teenagers and the availability of guns have turned life into a cheap commodity.

These are cross and switchblade communities where older people obtain a sense of cohesion through the churches, and with younger people it comes from the territory marked out by the gangs. South London's churches, and they are plentiful, probably have the largest and most God-fearing Christian congregations in the country. On Sunday mornings a tidal wave of humanity makes its way to worship, often past yellow police incident signs appealing for witnesses to muggings, assaults and stabbings.

These worlds don't co-exist, they collide in the murder of a 33-year-old woman shot dead at her niece's baptism service two years ago, or the apparently mistaken killing just over a week ago of the church-going Michael Dosunmu, who had just celebrated his 15th birthday.

Billy Cox's family appear to be among those good people living decent lives trying to do the best for themselves and their kids. His sister was heading for the Royal Ballet school, but somewhere in that fraught territory between childhood and maturity Billy slipped. At the time of his murder he was on probation for robbery and was fitted with an electronic tag to enforce a curfew. He was a member of the Clap Town Kids, part of the web of gangs that crisscross south London from the Poverty Driven Children in Brixton to the Blood Set in Streatham. Newspapers have him down as a drug dealer but friends defend his reputation. His devastated parents knew he was "not perfect but much loved". It may be that, like the majority of parents, they were naive about the reality of their children's lives.

Even the people who have tackled gun crime head-on in the past decade have been stopped in their tracks by the latest homicidal twist of events.

"This is a new phenomenon. We haven't dealt with a situation where the victims of gun crime have been so young," says Claudia Webbe, an adviser to the Metropolitan Police Operation Trident that tackles black-on-black crime and has had success in cleaning the streets of a generation of older criminals.

But there has been nearly a decade of work during which time the role model for young boys were crack dealers with flashy cars and big houses. These stereotypes lead to an easy theory, but offer no explanations as to why teenagers are killing each other with guns in south London. People point to the vacuum created in boys' lives by the absent father syndrome and schools where there are hardly any black, male teachers. The gap is filled by criminal glamour, by films that cast blacks as baddies, by music like the grime rap of south London that glorifies violence and guns.

Police intelligence says that older gang members are finding a way around five-year mandatory sentences for adults over 21 carrying firearms, by recruiting "youth sections" to run guns. The weapons are easily rented and passed around like portable status symbols. As little as £400 will deliver a rented "heater", a 9mm automatic pistol and "food", the ammunition to load it with, to the doorstep. In the gang jungle code of respect, where any perceived trivial slight has to be repaid with disproportionate physical retribution, the gun is the ultimate display of animalistic power.

"There is no one single rationale to why this is happening, but the weapon gives them power and respect," says Webbe. "Young people see the gun as a fashion accessory and because of the mix of youth and violence think they are indestructible, that nothing will happen to them , that they are above the law."

From the doorstep of Dosunmu's home, the locals can point out to Blakes Road and the stairwell where Damilola Taylor lay dying on a November afternoon in 2000. The building is long gone and in its place is a sparkling new sports centre, named after Damilola. An award-winning library have been built nearby and private houses and flats now sit next to refurbished council homes. But £290 million of regeneration can't rid the place of despair, a feeling of communal failure that comes from being stigmatised by the criminal activity of a few malign individuals.

The Reverend Nicholas Elder, whose church is just a short distance up the road, says: "It brought it all back, standing there at the top of Blakes Road, that feeling of hopelessness and pain. It's very sad to see people so distressed and fearful about their own future."

The black community in south London has been on a long march - from the "uprisings" of the 1980s, through the Stephen Lawrence inquiry, the Taylor murder trials, and the crack cocaine epidemic - to get back to this place, a point where teenage boys take guns to each other to prove their manhood.

Growing up in this environment is a tough challenge. Once a month Elder spends part of Sunday morning talking to the teenagers in his congregation, letting them confide in him the reality of life they protect their parents from.

"When you hear them talking about how they negotiate their way around the obstacles put in their path - the drugs, the alcohol, the gangs - you realise that coming of age around here is quite hard work," says Elder. "It demands an inner strength of them to get through."

Many children do make it through but lots get left behind. Unicef pointed out in a report last week that the UK is failing its youngsters by coming bottom of a league table for child well-being across 21 industrialised countries. Someone, other than the police, has to pick up the pieces.

"What we see is the emergence of young, black boys who don't believe that they have much stake in the future," says Uama Seshmi, a calm, centred individual who runs From Boyhood to Manhood, a project that rids the lost boys of their nihilism by giving them positive adult examples.

What Seshmi sees coming into the church hall he uses as a base for the project is a crisis of manhood. "There needs to be a change in the way society views young men," he says. "There needs to be an acknowledgement that boys grow up differently from girls. There has to be a recognition that the economy does not provide physical work that boys would do. Boys are being left behind, asking themselves what am I doing, where am I going?"

Seshmi is an antidote to the soul-searching over boys and guns and something of a candle against the darkness. He is the one who has to work out how to discipline the excluded schoolboy who spits in his face, he's the one left to wonder if that boy is going to be waiting around the corner at the end of the working day, so he is good on offering practical solutions.

Boys,heargues,willputthemselvesin danger, test their manhood against other men, and if education and the economy can't provide these outlets, then the risk of picking up a gun is what they will turn to.

Boyhood to Manhood runs on a shoestring but with admirable results in a crowded market of grant-writers and organisations that try to solve youth delinquency by giving kids what they want and not what they need. Trying to talk like them, dress like them, be accepted by them does not work, says Seshmi.

"A young boy doesn't want you to look like him, and, as a man, why do I want to stoop like that? He has to understand my responsibilities as a man, that there are certain things a man can't do and he has to see and understand these boundaries," he says.

Seshmi wants three teachers and a couple more "manly youth workers, men who are comfortable with their manliness", to show these boys an example. "With that I could do miracles," he says. Even small miracles would be helpful in this part of London.