About 36 properties have been flooded in south London as a water main burst near Herne Hill station.
London Fire Brigade said it was alerted to the incident at about 04:58 BST and was at the scene with Thames Water.
The burst is affecting three roads with Dulwich Road, Herne Hill and Norwood Road all partially closed.
Thames Water said: "We're really sorry to affected customers and we are working hard to put things right as quickly as possible."
The company said the 3ft-wide (0.9m) burst pipe had been located at the junction of Half Moon Lane.
It said: "Our main priority is to get people's lives and businesses back to normal.
"Our loss adjusters are en route and will soon be talking to people to get the insurance process under way.
"Customers should all still have water running through their taps as we have managed to re-route our network."
BBC London reporter Jonathan Savage, who is at the scene, said it was "wet and filthy" but the water was beginning to subside.
He said: "There's a bit of distress among some people.
"I've spoken to a newsagent who told me he's lost thousands worth of products because he had a large box of cigarettes on the floor, also I spoke to a butcher who's trying to figure out exactly how much damage has been done as he sweeps the mud and water off his floor."
A member of staff at the Fruit Garden greengrocers said the water was "knee-deep" and equipment had been damaged.
One local resident, Fran Graham, also tweeted: "Woke up to biblical scenes today, water level is dropping so hopefully I can get out of my flat soon."