Police are appealing for witnesses to a collision near Tesco in Old Kent Road on Friday evening to come forward.
Officers found a car, moped and coach had been involved in a collision.
They also found what they believe to be a firearm (a handgun) at the scene.
The driver of the coach stopped at the scene and was uninjured. There were no passengers on board.
The driver of the car had left the scene of the collision on foot, but returned and is uninjured.
The rider of the moped left the scene on foot.
Southwark Police are continuing to investigate the collision and where the suspected firearm came from.
They are appealing for any witnesses or anyone with information about the collision, or any related events before the collision, to contact them.
Anyone that can assist police is asked to call 101. Anyone wishing to give information anonymously can contact Crimestoppers on 0800 555 111.
The closure of Old Kent Road during Friday evening's rush hour caused major disruption to traffic and diversions to bus services.