Six fire engines and around 40 firefighters were called to a basement fire on Denmark Hill in Camberwell.
Part of the basement below a shop was damaged by fire. There were no reports of any injuries.
Station Commander Mark Nunan, who was at the scene, said: “On arrival, smoke could be seen coming from a flat roof and crews quickly assessed the scene to ascertain that it was coming out of a vent from a neighbouring shop.
“Everyone was out of the building before firefighters arrived and crews worked very hard to quickly bring the fire under control.
“Some congestion in the local area is expected this morning.”
The Brigade was called at 0810 and the fire was under control by 0919. Fire crews from Brixton, Peckham, Old Kent Road, New Cross, Lambeth and Dowgate fire stations attended the scene.
The fire is believed to have been accidental but the exact cause has been recorded as undetermined.