Scotland Yard is to nearly double its resources for fighting gang crime in a renewed crackdown on the problem.
The force has vowed to take a more joined-up approach to tackling gang crime in the capital.
Today hundreds of Metropolitan Police officers conducted a series of raids on properties in a bid to track down and arrest suspected gang members believed to be involved in crimes including assault, robbery and drugs supply.
At a press conference at New Scotland Yard, Assistant Commissioner Mark Rowley said that as of mid-morning, 109 warrants had been made resulting in 121 arrests for offences including the supply and possession of drugs and gang-related violence.
Half a kilogram of heroin and a "large amount" of cash were seized from an address in Southwark and a kilogram of heroin, crack cocaine and £10,000 cash was taken in Bexleyheath, with four arrests made there, he said.
The major operation, spearheaded by the Met's newly-formed Trident Gang Crime Command, marks what senior officers have described as a "step change" in the way the force tacklesgangs.
Scotland Yard revealed it has now committed 1,000 dedicated officers to fighting the problem, with the creation of the central Trident gang command and 19 new task forces to deal with local gang crime in problem boroughs across London.
Operation Trident was originally set up in 2000 to tackle gun crime in black communities across the capital and has gradually grown over the past decade.
Under the unit's expanded new remit it will retain the responsibility of investigating shootings, but also work alongside the borough task forces to proactively tackle gang crime, the Met said.
It will also have access to specialist resources including Operation Connect - the MPS unit set up to tackle violence driven by gang culture - and the Serious and Organised Crime Command.
There are an estimated 250 active criminal gangs in London, comprising about 4,800 people, mostly aged between 18 and 24, police said.
Of these gangs, 62 are considered as "high harm" and commit two-thirds of all gang-related crime, according to the Met.
The force added that gangs, ranging from organised criminal networks involved in Class A drugs supply and firearms to street-based gangs involved in violence and personal robbery, are responsible for approximately 22% of serious violence, 17% of robbery, 50% of shootings and 14% of rape in London.
Detective Chief Superintendent Stuart Cundy, commander for the new Trident gang crime command, said: "Since its launch in 2000, Trident has developed its expertise in combating shootings in London, and in recent years Trident has become more and more focused on tackling gangs across London.
"Today sees the Met building on the proven successes of Trident and other units.
"This new approach is a significant change for Trident as it now leads the Met's response to gang crime, but rest assured Trident will remain focused on preventing and investigating all shootings in London, regardless of the victim's or perpetrator's background."
By midday police said a further 35 warrants had been executed and 37 more arrests made.
Two people were arrested in Barnet on suspicion of possession of a firearm, and a handgun was recovered and two rounds of ammunition found nearby.
Around £4,000 was seized during the same raid and the suspects were further arrested for money laundering, police said.
Mr Rowley said the Met would spend £60 million on tackling gang-related crime this year and added that the crackdown would see officers taking a more "consistent approach" in tackling the problem.
"Part of it is about intelligence and tough enforcement," he said. "But we are under no illusion that enforcement will solve it alone."
Mr Rowley described gang members as being stuck on a "conveyor belt" of committing crime.
He said: "Our long-term solution is about trying to stop people getting on to the conveyor belt or, if not, get them off before they get too stuck into gang criminality, and that's where we're pleased with the response of local authorities who have already got good schemes, and all the borough leaders have committed to supporting us in this endeavour.
"What today's about is us raising our efforts and our tactics."
He said around 1,300 police staff were involved in carrying out today's action.
Speaking at the press conference, Territorial Policing Commander Steve Rodhouse said officers would increase their use of technology such as automatic number plate recognition (ANPR) cameras in the capital and use more injunctions to stop gang members communicating with each other.
"We know that gang members on their own are dangerous," he said. "But they are more dangerous when they are with their associates."
He added that the new operation would give officers more power to work with other agencies such as the UK Border Agency if a foreign national was found to be involved in a gang.
After the press conference, police officers, community workers and relatives of victims of gang crime came together for a photocall in Trafalgar Square, where they posed alongside Met Commissioner Bernard Hogan-Howe and London Mayor Boris Johnson.
Mr Hogan-Howe said: "This is a step change in how we tackle gang crime in London. It will allow us to identify and relentlessly pursue the most harmful gangs and gang members. It will help us identify young people on the periphery of gangs and work with partners to divert them away.
"Although we are now nearly doubling the number of officers dedicated to tackling gang crime, the police cannot do this alone. It is vital we work with the public, local authorities, charities and other agencies to prevent young people from joining gangs in the first place.
"If we can get some of them diverted to do more positive things great, but if they don't change their behaviour they get arrested.
"Today's about showing to the public and through the press how important it is to us. Our gang of seven and a half million has got to be bigger and better than the gangs who roam the streets of London.
"The hard work now starts. Today is just the launch, the hard work is day in, day out."
Barry Mizen - the father of schoolboy Jimmy Mizen who was attacked and killed in a shop in Lee, south-east London, while trying to avoid a fight in May 2008 - was also present.
He said: "My son's death was not gang-related but it was certainly violent crime among young people.
"I'm very encouraged by the proactive stance that's been taken by the Met to really try to get to grips and confront the issues of violent crime, confront the issues where young people are killing each other on the streets of London.
"We feel very strongly for obvious reasons that that needs to stop. We're not saying to people don't belong to groups - by all means we all need to have friends - but the violence has to lessen, there has to be less people losing their lives on the streets of London, and elsewhere for that matter.
Mr Johnson said: "We've been attacking gangs from all angles. Now the Met has a concerted and determined push to take out the ringleaders and tempt impressionable youngsters away from this destructive life."Nickie Aiken, Westminster City Council's Cabinet member for children, young people and community protection, said: "We welcome the Metropolitan Police's introduction of this dedicated squad, as well as their intention to work more closely with local authorities and other community groups.
"In Westminster we have seen a sharp rise in serious youth violence incidents over the past two years and have launched our own gangs strategy, Your Choice, to respond to the gangs problem now rather than wait until we become the next Hackney.
"A close working relationship with the police is a big part of the solution, particularly in terms of information-gathering, and local authorities have an important part to play in sharing gangs i ntelligence collected from local housing estates, schools and other community groups.
"But prevention is better than cure, and it's also important to look more widely at the root causes of gang recruitment among young people, which could include everything from stamping out domestic violence in the home, to providing tailored support for young people at risk of gang violence through our ground-breaking Family Recovery Programme."