London gangs with members as young as 12 are in "open warfare" - and the police have "lost control" of the streets.
These are among the damning claims made to Sky News following the shooting of a police officer in Hackney last week.
A recently retired sergeant who ran a gangs unit for the Metropolitan Police says up to 35 criminal outfits are operating in Hackney alone.
And it is not just Londoners who should be concerned.
Darin Birmingham, who earned 23 commendations during 30 years in the Met, claims gangs are going "unchecked" as they export Class A drugs to UK towns and cities as far away as Aberdeen.
He blames budget cuts, changes to stop and search rules, and the increased focus on terrorism.
"These gangs are in open warfare," Mr Birmingham told Sky News. "London is in peril and Boris should stand up.
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"The Met is losing about 50 good policeman a week. Within 18 months they're getting rid of the whole chief inspector rank to save money.
"I don't think there is enough quality in the police anymore and they've lost control of these gangs.
"The main thing for them is now radicalisation, people going to Syria. Everything domestic has been forgotten about and these gangs have become stronger."
Mr Birmingham - whose claims have been refuted by the Met - established Operation Aries in Croydon seven years ago to disrupt gangs who dealt in extortion, organised violence and hard drugs.
His tactic was to put "constant pressure" on gang members by interacting with them daily wherever they congregated - which in Croydon in 2008 meant the local chicken shops.
But the 51-year-old, who retired in 2013, feels recent changes in stop and search guidelines designed to protect the black community have made officers wary of engaging with local crews.
"If you go out and stop 10 people who are gang members and don't find anything you're likely to get a mark against your name saying you've only stopped people because they're black.
"A lot of the guys now don't want to get involved in stopping a BMW with four guys in the car - it's going to be a load of grief."
Mr Birmingham describes gangs as "like football teams with senior players and others as young as 12 coming through".
These children might be persuaded to sell cannabis after school, and before long school itself becomes an extracurricular activity.
It is a path that leads to trading crack cocaine and buying a firearm to protect your business interests.
"The older gang members start to s**t themselves," says Mr Birmingham.
"They're not worried about blokes their own age, they're worried about the young ones, the tinys. To get their rank they have to do something completely mad. They have to draw blood.
"These kids are chaotic. If they see two lads from another gang on the other side of the street they go over and stab them."
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The gangsters described by the former sergeant are fearless because of their youth and, sometimes, because they arrive in the UK from some of the most brutal parts of the world.
Mr Birmingham remembers arresting a teenager from Rwanda.
"He was 15 or 16 years old and he said he was in a mud hut with his family when the hutus came in, chopped his family to death and he hid under their bodies.
"He said to me, 'Do you really think I'm worried about being punched in the face in Croydon?'"
The new breed of gangster does little to affect the old order, however, with several long-established gangs retaining broader control of the streets.
"Some of these gangs have been established 20 years," said Mr Birmingham. "You've got the Tottenham Mandem, the Star Gang in Haringey; in Hackney you've got London Fields, Haggerston Fields. These are the people who control the smaller gangs."
In response to Mr Birmingham's claims, the Met told Sky News that budget cuts - amounting to £600m so far - meant there were "challenges".
But a statement added that murder, robbery and firearm discharges had fallen.
"Any suggestion that the Met is failing to tackle gang crime is an inaccurate reflection of the work being carried out on a daily basis by officers," the force said.
On stop and search, there was a recognition that a "balance needed to be reached".
A Home Office statement said: "This Government is also taking action against gang and youth violence.
"We are already working with 43 local areas facing problems with gangs and youth violence. This work includes supporting a network of over 80 experts with frontline experience of dealing with gangs and drug dealing linked to organised crime."