Four fire engines and 21 firefighters and officers are tackling a flat fire on Century Close in Hendon.
Part of a flat on the second floor was damaged by the blaze. An elderly woman heard her smoke alarm sounding and quickly left the flat. A further 15 people from neighbouring flats also evacuated the building.
Station Manager Sean Madeley said:
"The elderly lady has done exactly the right thing after hearing her smoke alarm sound which is to get out as quickly as possible. She was checked over by London Ambulance Service crews but has not been taken to hospital.
"Our advice is to have a working smoke alarm on every level of your home and the safest option is have the alarms linked together. This will give the earliest warning if there is a fire in your home."
Fire crews from Hendon and West Hampstead fire stations are at the scene. The Brigade was called at 1300 and the fire was under control by 1422. The cause of the fire is under investigation.