Rio's largest shanty town, patrolled by tanks, is open for tours, reports Michael McCaughan

Rio de Janeiro's offbeat tourist attraction used to be the great train robber Ronnie Biggs, who welcomed visitors to his elegant home for a small fee, seducing Europeans with a whiff of radical chic. Since the law caught up with him the gap in the alternative market has been filled by tours of Rio's favelas, with guides leading foreigners through volatile shanty towns that are normally off limits.

The day I visit Rocinha, the city's largest and most dangerous favela, the Brazilian government has authorised tanks and troops to back up police patrols outgunned by gangs with Uzis and hand grenades. The latest round of violence has hit the favela-tour trade hard, and I find myself alone at the visit's meeting point.

My tour begins at the foot of the favela, where three youths in yellow T-shirts guide me past a police roadblock and in to an abandoned building where Rejane Reis, owner of Exotic Tours, observes the neighbourhood from her rooftop. I join her and find myself eavesdropping on an English class given to a dozen favela residents studying for a tourism diploma. The students are selected not only for their personal skills but also because they are deemed "at risk", a euphemism to suggest that if they weren't here they might well be doing something illegal.

There are 800 favelas in Rio, three million people coming and going in a maze of narrow streets that are an ideal environment for illicit trade.

Reis explains that 20 students have left the course and temporarily moved out of the favela, staying with friends until the current round of violence has blown over. A Rocinha strongman called Lulu was recently killed by police, sparking a turf war for control of his lucrative drug network. "Things were OK when Lula ruled the roost," says Reis; the gang boss looked kindly upon the favela excursions.

Reis, a former airline stewardess, spent a dozen years working in the favelas before starting Exotic Tours, in 1998, combining traditional beach and wildlife outings with the equally exotic neighbourhood tours. As I listen to her a melody comes in to my head, refusing to leave until finally I visualise Johnny Rotten leering at me, spitting six words that would shake the conscience of the hardiest traveller: "Cheap holidays in other people's misery."

Most Brazilians detest the favelas and resent the outside world's fascination with them. "We have the most beautiful women, the best weather, the finest beaches and the cheapest beer," says José, a friend, "yet all you want to see is our shameful poverty."

Cariocas, as Rio residents are known, frequently cross paths with favela residents, who build their homes and nanny their children, but the lasting images are of beach invasions, when gangs descend from the hillsides and rampage across Ipanema, robbing middle-class bathers. Most Rocinha residents are rural migrants who have abandoned their homes over 50 years, building shacks on Rio's empty hillsides.

Reis is upfront about her motives for promoting the favelas. "I wanted to feel more comfortable about living in my own home and generate income for the people who live here." She appoints a guide for me: Marcos Mesquita Rangel, a chatty 30-year-old from Rocinha whose bright yellow T-shirt screams: "Please don't shoot the tourist." He talks me through the market, proudly showing off a community bank and Internet café.

He veers off to the right and climbs a steep staircase where front doors stand face to face, less than two feet apart. I turn sideways to allow someone to pass, glimpsing wedding photographs on a bedside table; favela architecture redefines the notion of personal space. I look up and see a tangle of telephone wires and makeshift electricity boxes, wires slapped together in thick overlapping bunches. There are no street names or house numbers, no planning permission and no building regulations.

One middle-aged man, with large silver rings on each finger and what seems to be half of El Dorado's lost treasure around his neck, points up at his home. "If you stick your arm out the window of my house you can touch God's balls."

Rocinha's unmapped interior is a catacomb of hallways that double back on themselves before moving upwards, always upwards, in a dense Legoland of bare-brick homes. It feels as if I'm climbing the stairs in a 40-storey building.

Rangel knows every nook and cranny, moving nimbly despite a wounded right foot. He says he was robbed and shot when he was a taxi driver. Most residents seem bemused by my presence; others turn surly when they see me.

Our next stop is a crèche perched on the top floor of a building where the risk of stray bullets has forced the staff to devise an indoor playground, complete with swings and climbing frames.

Finally, we reach Rangel's house, where I step gingerly over a 10-foot drop between the door and the hall. Inside it feels like any middle-class Rio home, with comfortable armchairs, television, bathroom, kitchen, family portraits and bookshelf. Rangel begs me to call it a day. I need no persuasion after the exhausting climb. I stagger dizzily out of the favela and within five minutes find myself among the jazzy boutiques and glitzy galleries that dot the Copacabana boardwalk.

That damned song still won't go away, but my previous images of gun-toting teenagers have been replaced by crèche kids slurping pasta and carrot soup and a rooftop English class where a generation of youths battle the odds to improve their lives.