London Fire Brigade said 10 fire engines and around 70 firefighters are tackling a fire at an electrical substation on Nestles Avenue in Hayes.
A transformer within the substation is alight.
Assistant Commissioner Pat Goulbourne said: “This is a highly visible and significant incident, and our firefighters are working tirelessly in challenging conditions to bring the fire under control as swiftly as possible.
“The fire has caused a power outage affecting a large number of homes and local businesses, and we are working closely with our partners to minimise disruption.
“Firefighters have led 29 people to safety from neighbouring properties, and as a precaution, a 200-metre cordon has been established, with around 150 people evacuated.”
Those that live nearby have been advised to keep their windows and doors closed.
The cause of the fire is not known at this time.
