Four fire engines and around 25 firefighters tackled a fire on Distin Street in Lambeth.
Most of a maisonette on the ground and first floors of a four-storey residential block was damaged by fire. One man left the maisonette before firefighters arrived. He is being treated by London Ambulance Service crews for smoke inhalation. Around 20 other people left the building before firefighters arrived.
The Brigade was called at 0803 and the fire was extinguished by 0902. Crews from Dowgate, Soho and Old Kent Road fire stations attended the scene.
The cause of the fire is under investigation.