Six fire engines and around 40 firefighters were called to a fire at a derelict bank being used as a squat on Morden Road in Wimbledon.
Part of the first floor was damaged by the blaze.
Station Commander Stephen-Remell Coleman, who was at the scene, said: “Around 17 people left the building before fire crews arrived. Thankfully there were no injuries.“
The Brigade was called at 1436 and the fire was under control by 1543. Fire crews from Wimbledon, Mitcham, Tooting, Wandsworth, Norbury and West Norwood fire stations were at the scene.
The fire is believed to be accidental and caused by a duvet left too close to a halogen heater.