We are in Scampia, a working-class district on the northern outskirts of Naples controlled by the Camorra – the city’s mafia clan. It is early afternoon on an ordinary weekday, but business is already brisk for the neighbourhood’s drug traffickers. Several lookouts, young men on powerful scooters, stand guard outside the lyrically named Le Vele – the Sails, three monstrous concrete tower blocks.

Skilled at spotting undercover police and members of rival clans, the lookouts exchange hand signals and yell out commands. As if at a checkpoint in a war zone, they stop unfamiliar cars and question newcomers. Beyond this first ring of steel, more sentinels guard one of the building’s entrances. Only a short drive away from the city-centre pier, where tourists board ferries to the idyllic island of Capri, the basements of Le Vele are crammed with mounds of uncollected rubbish picked at by stray dogs.

This sprawling maze is a beehive of hugely profitable activity. Beyond the lookouts, deep inside the building’s bowels, drug traffickers employed or controlled by the Camorra openly sell cocaine, heroin, ecstasy pills and hashish to a constant flow of hardcore addicts who queue to buy their latest fix. According to investigators, this is Europe’s biggest drugs supermarket: more is sold at street level here than in any other part of Europe. Local police say it’s a business worth half a million euros a day. Drawing on testimony from Camorra turncoats, investigators say that together with wholesale trafficking, up to 5,000 kilos of cut cocaine are sold there every month, for a street value of around €350m (£260m). Naples, 140 miles south of Rome, is among the top Mediterranean heroin and cocaine junctions.

Despite the alarmed rhetoric of local and national politicians, nobody seems able to control it. In stairwells, traffickers hold black pouches bulging with drugs and cash and ply their trade around the clock in eight-hour shifts. The business is kept running day and night and ceases only briefly whenever a police patrol car is spotted. The complex even houses a siringaio – a man who peddles syringes alongside chocolate bars, soft drinks and takeaway espresso.

Unperturbed, addicts shoot heroin in broad daylight in Le Vele’s vast basements and along a nearby roadside ditch, where it is not uncommon for locals to stumble across bodies of overdose victims. Incredibly, the district’s Carabinieri headquarters, one of Italy’s three police forces, is less than 100 yards away.

In another Camorra-run piazza, as drug- dealing patches are called, buyers gather to shoot up in a one-storey disused kindergarten in the midst of a housing estate teeming with local residents. At any one time the abandoned building is crowded with up to 200 addicts.

The floor of the derelict and damp hall is splattered with blood as people help each other to inject heroin in the neck, legs, arms and even groin. The adjacent rooms, heavy with the stench of disease, are carpeted with layer upon layer of used syringes and bloodied needles.

Amid the filth and degradation, surreally, one addict sweeps the floor incessantly with a tattered broom, muttering to himself, while outside, young lookouts with hardened faces stand guard to protect the day’s trade. Long inured to such scenes, a group of children come out to play football in the street, while above them, housewives hang their laundry.

The lucrative business is entirely in the hands of Camorra clans, a vast network of organised-crime families and syndicates from the region of Campania, of which Naples is the capital. While less famous than its Sicilian counterpart, increasingly the Camorra is said to be no less powerful than the Cosa Nostra mafia.

Given its myriad clans – at least 78 in and around Naples alone, according to the latest official count – the Camorra is more fractious and prone to bloody turf wars than its centralised Sicilian big brother. In Naples’ city centre, a labyrinth of noisy and colourful side streets, local clans kill each other’s men over control of a single alley. As a result, Campania’s murder rate is said to be the highest in Europe – an estimated 3,600 in the last 30 years. Last year, the city and its immediate surroundings witnessed 116 Camorra-related killings – one every three days.

Drug trafficking is only one of the Camorra’s businesses. Like Cosa Nostra and the ’Ndrangheta, their counterpart in the region of Calabria, the Naples clans are also into loan sharking, construction, industrial waste disposal and racketeering. Estimates of their profits are sketchy, but according to a recent report by Italy’s national retailers association, the country’s mafias earn a staggering €90 billion a year – £67 billion – making them Italy’s biggest business.

The greater the earnings, the more brutal the clan wars. So when, four years ago, a conflict broke out over control of Scampia and Secondigliano’s drug trade, 60 contract killings followed in just a few months. Open war is over but, like aftershocks in the wake of an earthquake, revenge hits are still taking place and so far the clash has claimed around 90 lives.

Times Recommends

• Sceptic challenges guru to kill him live on TV
• Taxpayers foot the bill for Zuma's lifestyle
• Oprah Winfrey meets her match