A sub-machine gun and 70 rounds of ammunition were seized by police during a firearms raid in Mitcham.
Officers recovered the Scorpion sub-machine gun and bullets after executing a firearms warrant at a house in Commonside East at approximately 11.50pm on Friday night.
Nine people were arrested on the property, includng six men and three women aged between 18 and 54.
Police used a Taser on one woman during the raid and she was taken to hospital but has now been discharged, arrested and is in custody at a south London police station.
Officers also discovered two magazine clips, £3,000 in cash and a large quantity of cannabis at the house.
All nine have been arrested on suspicion of firearms and drugs offences and remanded in custody.
Detective Superintendent Damian Allain of Trident Gang Crime Command, which investigates gun related crime in the capital, said: "This is an extremely dangerous weapon capable of shooting 850 rounds a minute.
“In the wrong hands this firearm is deadly and is now safely off the streets of the capital.”