Water levels have subsided following the mains burst on Half Moon Lane in Herne Hill which left fire crews using boats to navigate waters up to 1.5 metres deep.
London Fire Brigade said crews were working to pump water out of the flooded basements of 37 homes and businesses. It warned residents to remain in their homes for safety.
Bruce Grain, Brixton fire station manager, said: “At its peak the water was up to a metre and a half deep and covered an area of about 200 metres by 200 metres, but fortunately it has now subsided although we will be there for most of the day helping to clean up the area.
“The basements of 37 homes and local businesses have been confirmed to have been flooded with water up to three metres deep and our crews are working really hard to pump out the water and minimise the damage.
“The pub nearby has around 60,000 litres of water in the basement alone and should take an hour and a half to clear with our high-volume pump.
“At the same time we’re spraying the road to get rid of the mud so roads can be reopened and people can get on with their business.”
The Environment Agency is also at the scene, the LFB said, along with local authority staff and National Power, which has turned off electricity in the affected area. The gas board is also attending because of the smell of gas, it added.
One resident, Fran Graham, tweeted: “Woke up to biblical scenes today, water level is dropping so hopefully I can get out of my flat soon.”
Craig Rance, a spokesman for Thames Water, said: “We’re really sorry to affected customers and we are working hard to put things right as quickly as possible.”