Six fire engines and around 40 firefighters were called to a fire at a flat in Lawn Road in Belsize Park.
Most of a flat on the 10th floor of the building was damaged by fire.
Three people were led to safety via an internal staircase by firefighters wearing breathing apparatus and were treated at the scene for smoke inhalation by London Ambulance Service crews.
The Brigade was called at 1220 and the fire was under control at 1316. Fire crews from Kentish Town, West Hampstead, Euston and Holloway fire stations attended the scene.
The cause of the fire is under investigation.