Eight police raids across west London have struck a "massive blow to an established gang", the Metropolitan Police said.
A submachine gun, ammunition, cash and suspected Class A drugs were seized in the raids in Northolt, Greenford, Fulham and Brentford.
Nine people - including a 14-year-old boy - have been held on suspicion of drug and firearms offences.
Det Insp Driss Hayoukane claimed the "MDP" gang had "corrupted children".
During a search in Stephendale Road, Fulham, officers found a Skorpion submachine gun, along with another handgun, 40 rounds of ammunition, and a kilo of suspected Class A drugs, Scotland Yard said.
Overall, six males and three females, aged between 14 and 49, were arrested in the early hours of Thursday and are now in custody, according to the Met.
The operation was the result of about six months of planning by the Met's anti-gang Trident unit, the force said.
Metropolitan Police Commissioner Cressida Dick had been on patrol in Hackney on Wednesday evening, where she said gang tensions were high after a spate of stabbings in recent weeks, including the killing of 18-year-old Israel Ogunsola.
She said: "They are very violent, several of them have a history of serious violence, at least one is suspected of regularly using a firearm.
"They will be arrested - not only have they been, as it appears to us, supplying crack cocaine and heroin, they've been making a huge amount of money.
"They've been exploiting vulnerable people and very young people have been engaged in the drug-dealing operation. So they need to be locked up."
Asked if raids had been stepped up following a recent rise in violent crime, Det Insp Hayoukane said: "It's business as usual for us, our unit works to capacity all the time.
"What we have taken out is probably a line which has been supplying the Earl's Court and Fulham areas.
"This gang have been running that line for quite a while and making a lot of money.
"These gangs have corrupted children and are using them to ferry drugs.
"If you look across London these are the kids that are getting involved in violence and stabbings because they are generally on the streets while the suppliers are removed from it, collecting the cash."