Four fire engines and around 25 firefighters were called to a fire at a restaurant with flats above on Station Lane in Hornchurch.
Half of the ground floor and part of the first floor of the building were damaged by fire. There were no reports of any injuries.
A London Fire Brigade spokesperson said: "This incident shows just how important it is to have working smoke alarms fitted inside your home. Smoke alarms alerted a member of staff who was asleep in the flat above the restaurant to the fire. They then alerted other occupants and left the building before calling 999.
“Smoke alarms give the earliest possible warning when a fire starts, providing time to escape, and we would urge everyone to make sure they have one fitted in every room where a fire can start except kitchens or bathrooms where heat alarms are more appropriate.”
The Brigade was called at 0745 and the fire was under control by 0848. Fire crews from Hornchurch, Dagenham and Harold Hill fire stations attended the scene.
The cause of the fire has been recorded as undetermined.