Six fire engines and around 40 firefighters were called to a gas leak in Beddington Lane in Croydon.
One gas main was ruptured. Firefighters carried out a sweep of the area and assisted gas engineers.
Approximately 450 people were evacuated from the immediate area as a precaution and a cordon is in place.
Drivers are advised to avoid the area if possible as local road closures are in place.
The gas leak is not connected to the fire further along Beddington Lane.
The Brigade was called at 1237 and will be a protracted incident with crews remaining at the scene into this evening. Fire crews from Bromley, Mitcham, Norbury, Wallington and Purley fire stations are at the scene.