“We are trying to disrupt things,” ­Couling says. “We want them to know they will be searched. If we take one knife off the street we could save ­someone’s life. We sweep gardens, parks; we look in bins. We find a lot of weapons, including knives taken from parents’ kitchens.”

Couling operates in an area of north London that has 14 gangs and where five fatal stabbings took place this year: the most recent last month when Marcel Campbell, a father of two, was knifed to death at 6.30pm outside an ice-cream parlour on Upper Street, Islington, less than a mile from the home of the foreign secretary, Boris ­Johnson. Campbell’s family have denied he was in a gang.

Last week, The Sunday Times joined her patrol. On her patch, known as ­central north command, multimillion-pound houses stand next to council estates. The area includes Islington and Camden, where middle-class tourists go to the bohemian market at weekends, some to buy drugs from street dealers.

First comes an intelligence-led briefing at the main police station. Details of gang members, aged from 15 to 25, and maps of the postcodes in which they operate flash up on electronic whiteboards. Green lines mark “alliances” among groups such as Cally Boyz and Essex Road; red lines are “feuds”.

A web search of gang names that have been linked to the area reveals horrifying levels of savagery. Last autumn, a member of the Essex Road gang who stabbed another teenager in Newington Green was jailed for 14 years. He plunged a blade into the chest of the victim, who like him was 16, apparently motivated by revenge.

Two members of the Cally Boyz gang were convicted this year of stabbing to death a 27-year-old suspected of talking to police. Five masked men on bikes chased down Nashon Esbrand and attacked him in a shop.

Another gang must go unnamed — “They see it as a badge of honour to get into the press,” says a police source — but five members were jailed for smash-and-grab raids on mobile phone shops in­volving hammers, knives and a gun.

It is 4.45pm. Pictures of suspects flash up on the whiteboards. A group of teenagers on bikes has been seen on a road, carrying knives and wearing balaclavas. A youth with a weapon has been spotted in a park; police have called his parents. An image of the suspect for the Upper Street murder appears, with a warning not to approach him “proactively”.

Once the briefing is over, two vans full of police officers swing out of the ­station, sirens screaming. A suspect has been spotted in Hornsey Road. Stopped with his girlfriend and thought to be in breach of a restraining order, he does not run. He is not carrying a knife. A driver berates them for harassing a black man.

Couling spots a drug deal taking place at the back of a council walkway. The dealer escapes on his pizza delivery moped, but the buyer, a young Italian man, is surrounded and cuffed. He admits paying £80 for four wraps of cannabis. “I’m not a criminal,” he pleads. He is cautioned and released.

Not far away, another suspect is spotted on the street. He rides his bike into the traffic and gets away. In the van there is gentle ribbing. Couling is pitting two young officers against one another. Who is fastest? Who has caught more today?

A young man going into a council estate is searched next: he has been ­recognised as someone who owns knives, though he is not carrying one today. “Have a good one,” says the officer. “Jog on, you bastard,” the man replies.

Gang violence in north London is not new. Two years ago, Stefan Appleton, 18, was stabbed through the heart with a machete in a playground in Islington as rival gangs battled each other in a turf war. Today, though, it is more frequent. On Thursday night in Brent, north London, a woman was fatally stabbed, a man in his twenties was riddled with bullets from a machinegun and another man was critically injured after being stabbed.

Youngsters are also being drawn in, and another initiative involves police working with schools and other agencies to identify children as young as seven thought to be at risk of being lured into delivering drugs for gangs.

We tour the back of peeling concrete council estates, stopping by tiny playgrounds. It is eerily quiet. “Someone’s put the word out we are about,” says Couling. A group of young women wave from a pub bench. “Someone likes us,” says an officer, sounding almost relieved.

Couling’s reassurances about the safety of London’s streets were backed by her boss, Nick Davies, the commander of the central north London area. “Sergeant Couling is correct,” he says. “Gangs are about control of territory and control of drugs. Broadly speaking, it is rare someone just going about their ordinary business would be involved in a ­stabbing.”

Sometimes knives are used to threaten people in street robberies, Davies said. He added: “The streets are safe — and it’s because of everything we are doing.”

A few days after the patrol, the police arrest and charge a 21-year-old man for the murder on Upper Street. He is the man named at Couling’s briefing.

In Scotland, two police officers are in a serious condition in hospital after being stabbed while responding to a 999 call.

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