A police report on London's gang culture has identified 169 separate groups, with more than a quarter said to have been involved in murders.
Three teenagers died in shootings in south London in two weeks
Gangs are responsible for more than a fifth of youth crime in London, according to the Metropolitan Police report seen by BBC London.
The unpublished report found that nearly half of London gangs had been involved in serious assault.
Three teenagers have died in shootings in south London in two weeks.
The largest number of gangs are said to be in Hackney, east London (22 gangs); Enfield in north London (13); Lambeth and Merton in south London (12 gangs each); Waltham Forest in north east London (11) and Brent in north west London (11).
Gangs were found to have around 20 to 30 members, and to commit crimes in smaller groups of three to six.
African-Caribbean gangs were described as the largest group, followed by south Asian and white gangs.
Religion was also found to be a defining factor, with some gangs comprising solely Muslim or Catholic members, for example.
The report distinguished between 19 gangs considered responsible for a high level of harm and 29 regarded as using a medium level.
A former gang leader said some young people became gang members in order to feel protected.
"They feel safe within a gang because you have got older people in the gang who are always going to look after you," he said.
"You are always moving as a pack. It's as you fight in a war.
"If you are fighting in a war you are not going to send just one soldier out. It's definitely a war out there."
Criminologist Dr John Pitts, from the University of Bedfordshire, said: "There are probably no more than 1,500 to 2,000 young people in gangs in all of London, but their impact is enormous."
Dr Pitts said the influence of gangs could determine which schools and colleges young people felt able to go to.
"There are quite a number of youth workers in London boroughs who are finding it impossible to run a service," he added.
Fifteen-year-old Billy Cox is the latest victim of violence blamed on gangs.
He was shot in the chest at the family home in Clapham, south London.
Metropolitan Police Commander Paul Minton said: "There does seem to be evidence of a rise in the number of gangs and there seems to be an increase in the number of young people involved."
However, he said the south London community had been "reaping the effects" of increased police activity and armed patrols in the area in the wake of the recent shootings.
"The series of incidents that have occurred in south London have been unprecedented in our experience," he said.
"As a consequence we are doing a great deal to ensure we are tackling the criminality that is occurring there."