London Fire Brigade say that a man escaped from a burning tyre workshop before firefighters arrived on the scene.
In addition, some local buildings in Kensal Rise have been evacuated.
Parts of the building's roof, first floor and ground floor are alight but no injuries have been reported.
Due to the acetlylene gas cylinder, crews have set up a hazard zone of 100 metres and are spraying water onto the cylinder to cool it down and make it safe.
Harrow Road is currently closed between College Road and Scrubbs Lane.
The Brigade was called just after 3pm and four fire engines and 21 firefighters and officers from Willesden, North Kensington, Paddington and Park Royal fire stations are at the scene.
Crews are expected to be at the scene for several hours this evening.
There are now six fire engines and 35 firefighters at the Kensal Rise workshop fire.
Crews are tackling the flames and cooling the gas cylinders down to make them safe.