Three years ago, a group of street children sleeping on the steps of Candelaria church in Rio de Janeiro were fired on by police. Eight of them died.

The case has just come to court, with one - just one - officer being sent to prison. Liberals have been horrified by this leniency, but the general feeling in Brazil is more one of astonishment - that the law was successfully invoked at all. Street kids are murdered in Brazil at an average rate of four a day, a cheap form of life, a sort of vermin. They simply don’t count. This is the theme of a remarkable play to be performed at the Young Vic in London next month, which puts first hand experience of its raw material through the fires of crusading passion.

Director Marcio Meirelles and the Bando de Teatro Olodum hail from the Brazilian city of Salvador, and their play Ere is rooted in the social and spiritual realities of life in this tropical port. Salvador was Brazil’s original capital, founded by the Portuguese, whose glorious architectural legacy is everywhere intact. But their social legacy was less glorious: this was a city of slaves imported from Nigeria, Dahomey, and the Congo, and though civil rights are now theoretically universal, anti-black discrimination is still rife.

And so, defiantly, is black religion. The Catholic Church has tried for centuries to stamp it out, but the Yoruba candomble religion, with its trances, sacrifices, and ritual music and dance, has thrived unstoppably. Indeed, it has even made converts from “white” religions, and preserved rites which are now extinct in their African country of origin. What most enrages the Catholic Church - or at least its conservative wing - is the fact that Salvadorean candomble has purloined its Christian saints, and pressed them into service as Yoruba deities. Outrageous!

Ere is the Yoruba name for the transitional state between spirit-possession and normality: the candomble gods stalk through Olodum’s play like avenging deities. But Ere’s key characters - Cosmas and Damian - are mythical twins who in their Catholic incarnation are celebrated every autumn in the streets of Salvador. As the protectors of lost children, they are honoured by a ritual in which food is distributed to all the children of each neighbourhood: their African counterparts, the Ibeji, have a similar ritual background. Meirelles and his actors have devised a plot - on the wings of song, dance, and drums - in which these twins become the healing antidote to the image of the slaughter at Candelaria.

But they have abundant local material to draw on: Salvador has an estimated 1,200 children - many as young as four or five - who sleep on the streets, with ten times that number inhabiting the streets by day. You see them on every corner, some resourcefully cleaning car windows, some peddling holy ribbons, but many lurking in groups with an eye on the chances of picking a pocket. Some are dazed with drugs - sniffing-glue, or cough syrup injected into the veins - and many look sick. The girls are often prostitutes. The police are warier than they used to be, but they still haul children in for “questioning”, after which their victims are spat back into the street, black and blue with bruises.

Rejane Maria, one of Olodum’s actresses, smuggled me into the juvenile penitentiary where she does her day-job, so that I could get a whiff of the reality the company are trying to convey. A bleak concrete complex, with unglazed windows and a maze of locked doors: a miserable place, yet a home to which its young inmates often strive, through intentionally discovered misdemeanours, to return. Rejane’s group are due to perform a play they have devised: the subject is the dispossessed Brazilian Indians, an underclass even more put-upon than they.

We sit cross-legged on rush mats as paint-daubed warriors chant and wave their spears at us: the guard on the door looks at his watch. Some of these thespians are murderers, prone to sudden outbursts of aggression: for everyone’s safety, they should not be allowed to brandish their weapons too long. The play over, the actors crowd round the unfamiliar white face: where am I from? “Inglaterra? Ah, off the map” sighs a girl wistfully. They all want to show me their rooms: the girls’ are spotless and bare; the boys’ all sport home-made altars, round which they have placed the votive objects they most value - tubes of toothpaste, sticks of deodorant. One altar is topped by something oddly familiar - a statuette of Cosmas and Damian, protectively flanking a small child. Even here, Catholicism and candomble are syncretised. Rejane introduces me to a stocky youth on whom she has modelled her character in Ere: he’s become a pillar of respectability in prison, but was put in here for murdering a robbery-victim with an axe.

She then takes me back to the favela - shanty-town - where she lives: these improvised encampments are the cradle of the street-kid army. It’s a cheerful place, but raw sewage runs through its central ravine, by which pigs and children loiter as though in a mixed-species zoo; the smell is asphyxiating. When I ask the favela’s director of communications what effect this has on the children’s health, he rolls his eyes in exasperation. Fevers and diarrhoea: and there’s no doctor on site. Frankly, if I were a child here, I’d take to the streets.

But elsewhere things are being done, and projects set up, to alleviate the problem. The most notable of these is called Axe - after the candomble word for life-force. Axe is the brainchild of an Italian United Nations Children’s Fund official who decided to apply social-work strategies in this extreme situation, and gathered a group of like-minded idealists ready to work for peanuts, and to brave the inevitable hostility of their quarry. Street kids in Salvador are wary of adults who approach them with offers: abduction, slavery, and even murder can result. Axe workers spend weeks discreetly trailing their targets: they wait for the children to come to them, and only then come out with their educational suggestions.

One Axe worker I met is an Italian fashion designer who came on a package holiday three years ago, got enthused by what Axe was doing, wound up his company, and has stayed in Salvador ever since. He has turned his expertise to good effect, teaching Axe kids tailoring, and getting them to run their own boutique. Other Axe projects include a circus course and a paper-cutting workshop. Arts education was seldom harnessed to such desirable social ends. Yet other Axe kids are now formally learning capoeira dance: this extraordinary martial-arts form, which is all a matter of spinning kicks from a handstand position, is now becoming the rage among young blacks in London.

Meanwhile, the big guns of showbiz have also discovered Salvador, and, more specifically, the drummers of Olodum. Paul Simon first spotted them, and used them on his album The Rhythms of the Saints (Yoruba saints, of course). This year Michael Jackson launched a video in which he cavorts with Olodum’s drummers in Salvador’s favelas and on the cobbled streets of the old Portuguese quarter. Olodum are a local carnival band, and they plough their record industry earnings into social work, into coaching junior drummers and into the theatrical venture led by Meirelles.

And while Catholicism sits on the sidelines, candomble rolls its sleeves up and gets to work: the candomble settlement I visited had its own thriving school, the local equivalent of a C of E school in Britain. Its teachers earn l00 US dollars (about Pounds 66) a month; their classrooms are devoid of the most basic equipment, but they still manage to give their pupils an educational start in life, and a code to live by. And these pupils are an exuberant lot: the moment I pointed my camera at them, they all gave me back the “heavy metal” sign.

Meirelles had initially considered staging Ere with the street-kid actors he knows, but bringing little outlaws on international tour would have been unthinkable. “The actors who play our fictional children won’t act like children,” he says. “But then street kids don’t either. They have guns by the time they are six: they have no proper childhood.”

This play may not give that luxury back to the present generation, but it may sow the seeds for a better future for the next.

Ere is presented as part of the Out of Lift Festival. Young Vic June 20-22, with a schools matinee on June 20. Tickets: 0171 928 6363