Eight fire engines and around 60 firefighters were called to a fire at flats above a shop on Pinner Road, Harrow.
The second floor and roof were destroyed by fire. One man was assisted from the first floor and one woman and a baby were assisted from a second floor flat by a passerby before the arrival of the Brigade. There were no reports of any injuries.
Group Manager Tim Frost, who was at the fire, said: “We would like to praise Mick O'Beirne, the man who assisted the people from the flat, for his courageous actions.
“He saw the flames coming from the roof and forced the front door, without any concern for his own safety, before assisting a man from the first floor flat and a woman and baby from the second floor.
“He did a tremendous job and without his swift actions and quick thinking, this incident could have had a very different outcome.
“When crews arrived they gave first aid and tackled the fire which was swiftly brought under control and damage was mainly confined to the roof space.
“While what Mr O'Beirne did was brave, we want members of the public to stay safe and would always encourage people to call 999 first in the event they witness a fire so we can get there a soon as possible.”
The Brigade was called at 2209 and the fire was control at 0037. Fire crews from Northolt, Ruislip, Harrow, Stanmore and other surrounding fire stations attended the scene.
The cause of the fire is under investigation.