Cars were crushed after a burst water pipe flooded an underground car park.
A number of vehicles crumpled after polystyrene insulation under the floor appeared to float or swell, forcing them into the concrete ceiling above.
Flood water rose several feet at the newly-built car park below a block of flats in Crayford, south-east London.
Residents said they had been warned if any of the cars were removed too hastily it could result in gas explosion.
Between 12 and 16 vehicles are thought to have been damaged.
One owner was told her car was written off as a result of the damage and might not be covered by insurance.
Sadia Farzana Chitra, 42, said: "My car is crushed, the roof is gone.
"We were told we cannot move it because there is a possibility with the pressure of the car there could be a blast because of the gas inside.
"It can blow up the whole building."
About 5,000 homes were left without water and Thames Water said its team was working to fix a 24-inch pipe.
A spokesman for Gallions Housing Association, which manages the car park, told the Evening Standard: "All Gallions residents are safe, have clean water and there has been no water ingress into anyone's home.
"Water has damaged some cars in the car park in the basement, and Thames Water have advised that they will have officers based at the library tomorrow [Monday] to get all of the details from those affected."
The flooding caused severe travel disruption after filling the road near Townhall Square.