Officers discovered the weapon after ordering two cars driving in convoy to pull over in Crossthwaite Avenue on Monday 15 November.
They searched the vehicles and found a PM63 RAK machine pistol, capable of fully automatic fire, hidden in a speaker on the rear seat of one of the cars.
The firearm was made safe and has been sent off for testing.
The drivers of the vehicles, aged 31 and 53, were arrested on suspicion of possession of a firearm with intent.
They were taken to a south London police station where they remain in custody.
Officers seized the weapon following an intelligence-led operation by the Met’s Specialist Crime Command.
Detective Victoria Sullivan, from the Met’s Specialist Crime Command, said: “Excellent investigation work by officers has led to an extremely dangerous weapon, which has been designed to cause serious harm or death, being taken off the streets of London.
“We remain dedicated to making London a safer place for everyone by carrying out operations such as this one. We will continue to disrupt those intent on committing violent crime on our streets.”
Detective Daniel Smith, from the Central South Command which covers Lambeth and Southwark, said: “I know this lethal weapon being recovered on the streets of Southwark will cause concern for local residents. But let me assure you that firearms have no place on our streets and we are doing everything in our power to arrest those who carry and supply guns.
“However, we cannot tackle this issue alone and we rely on information from the public to help us to robustly tackle gun crime. Any information you have, no matter how small, can really help our investigations."
Anyone with any information should contact police on 101 or Tweet @MetCC.
To give information but remain 100% anonymous call the independent charity Crimestoppers on 0800 555 111 or visit crimestoppers-uk.org.
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