Five fire engines and around 30 firefighters and officers were called to a fire at a flat above shops in The Concourse in Colindale.
Part of a flat on the fourth floor was damaged by fire.
One woman and four children left the flat before the arrival of the Brigade. A further 20 people left the building before firefighters arrived.
The Brigade was called at 1855 and the fire was under control at 2103. Fire crews from Mill Hill, Hendon, Stanmore and Finchley fire stations attended the scene.
The cause of the fire is believed to be due to a curtain catching alight.