Police have said they could not say whether the shot was fired at one of the robbers or at their vehicle, and they had yet to determine how much money was stolen.
A police spokesman said: “Two men have been arrested by Flying Squad officers following a cash-in transit robbery in south London.
“At around 2.30pm, Flying Squad officers were in Portland Road and witnessed a robbery of a cash delivery to a petrol station.
“The officers took steps to arrest the suspects, two men. During the course of the arrests a police firearm was discharged by an officer from the Flying Squad. No one was injured.
“The two men fled the scene in a vehicle. The suspects’ vehicle then collided with a vehicle driven by a member of the public in Grange Road.
“The driver of the car, a 70-year-old man, was injured in the collision and has been taken to a south London hospital. His injuries are not believed to be life-threatening.
“A 27-year-old man was arrested at the scene on suspicion of robbery. A 26-year-old man fled the scene and was found by officers in a property nearby.
“A Taser was discharged by Flying Squad officers during the course of the arrest of the second suspect.”