At 1.41am on Monday, 23 January police were called to Charlton Church Lane, SE7 following reports that a group were trying to break into a shop.
The group had left prior to officers arriving and without successfully gaining entry.
At 2.02am officers stopped a car in nearby Nadine Street.
Inside the car they found a firearm.
Four people were arrested on suspicion of possession of a firearm and attempted burglary.
They remain in custody.
A cordon is in place in Charlton Church Lane while further enquiries are carried out.
