As many as 60 firefighters were said to be on the scene, but the cause of the explosion was not immediately known.
Firefighters were dealing with a blaze in a ground floor flat in a three-storey building.
A spokesman for Scotland Yard said: "Police were called at 17:14hrs on Monday, 23 January to reports of an explosion at a block of flats in Ardleigh Green Road, Hornchurch.
"Officers, the London Fire Brigade and the London Ambulance Service are currently at the scene. We await an update as to the number of people injured.
"At this early stage officers are unable to confirm the cause of the explosion but enquires are underway. Residents are in the process of being evacuated from the building. The Local Authority has been informed. Local road closures are in place. Motorists and pedestrians are advised to avoid the area."