People are believed to have died after a suspected gas explosion in a shop underneath flats led to the collapse of the building.
London Fire Brigade (LFB) said they believe there are fatalities within the property on King Street in Ealing, west London, but are unable to confirm how many.
Emergency workers are involved in a "complex" and "painstaking" operation as they continue to search for anyone who may still be inside after the incident on Wednesday.
Station commander Paul Morgan from LFB said: "Sadly we believe that there are fatalities within the property although we're unable to confirm numbers at this stage.
"The explosion caused substantial damage to the shop and structural damage throughout.
"It is a painstaking and protracted incident with firefighters working systematically to stabilise the building and search for people involved.
"We'd ask for people to avoid the area at this time."
He said a child and four adults were rescued in the initial stages, at the rear of the property, using a ladder, while a further fourteen adults and two children self evacuated from surrounding properties.
Firefighters have rescued a number of people from a suspected gas explosion and building collapse in #Ealing. Four adults & a child were rescued at the rear of the property by crews using a ladder. The search of the building continues https://t.co/MGbJhqHL1Hpic.twitter.com/HEEwqeCxF7
— London Fire Brigade (@LondonFire) October 21, 2020
Pictures taken behind the property show debris jutting out of the building.
LFB said they were called just before 6.40am, adding that six fire engines and about 40 firefighters are at the scene.
LFB's specialist urban search and rescue (USAR) crews are carrying out the operation.
A spokesman said the windows of the shop were "completely blown out" in the incident.
The Metropolitan Police said the incident is not being treated as suspicious and is believed to be a gas explosion.
The force said a large explosion was found to have taken place inside a shop and a man was found injured.
A London Ambulance Service spokeswoman said they have treated and discharged one person, and they are continuing to work with other emergency services at the scene.