Five fire engines and around 30 firefighters were called to a small fire at a block of flats on Lyon Road in Harrow.
The fire occurred in the electrical intake room situated within the basement of the building and it was safely dealt with by crews. Around 200 people evacuated the building before firefighters arrived and there were no reports of any injuries.
The Brigade was called at 2201 and Control Officers mobilised crews from Harrow, Northolt, Wembley and Stanmore fire stations to the scene. The incident was over for firefighters by 2340.
The fire is being treated as accidental.