Police found a handgun after searching a car they thought was suspicious near a busy road.
The firearm and a box of viable ammunition were seized from a parked van by specialist officers in Holmwood Road, Brixton Hill, on Tuesday.
Armed officers forced their way into the vehicle and a search resulted in the recovery of a Heckler and Koch handgun.
They also found a box with 50 rounds of viable ammunition.
No arrests have been made but enquiries are ongoing to identify and locate those responsible.
Detectives from the Met’s Specialist Crime Command are investigating.
Detective Sergeant Vanessa Calafat, from Specialist Crime, said: “I am pleased that we have been able to take a dangerous weapon, that could have been used to kill or seriously injury someone, off the streets of London.
“The fact that we found the firearm next to a box full of ammunition shows the dangerous motivation of those in control of it.
“We are working around the clock to bring those involved to justice.
“Firearms have absolutely no place on our streets and their removal makes London a safer place for all.
“The Met remains committed to tackling violent crime and seizing weapons such as this one.
“We will continue to crackdown on criminals and bring them to justice.”
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