Fifteen fire engines and around 100 firefighters tackled a fire on Rye Lane in Peckham.
Station Commander Craig Abbott, who attended the scene, said: "Firefighters worked hard in arduous conditions to bring this fire under control.
"A single storey extension to a department store on the first floor of a commercial building was destroyed by the fire. Around 50 people left the building before the Brigade arrived. There are currently no reports of any injuries.
"A significant amount of smoke was produced by the fire and local residents were advised to keep their windows and doors closed.
"Traffic in the area was also impacted and members of the public were advised to avoid the area."
One of the Brigade's 32-metre turntable ladders was used at the scene as a water tower to help fight the fire from above.
Control Officers took the first of 43 calls at 2054 and mobilised crews from Peckham, Old Kent Road, Brixton and surrounding fire stations to the scene. The fire was under control by 2350.
The cause of the fire is under investigation.