Thousands of people have been left without electricity after suspected vandalism caused major fire damage to high voltage electricity cables.
Properties in Dartford in Kent and Welling, Orpington and Bexleyheath in south-east London lost power supply at 1247 BST, EDF Energy said.
Engineers restored power to some areas but it may be another 36 hours before electricity is restored to all homes.
Darent Valley Hospital in Dartford has also been affected by the power cut.
There is sufficient power to run the whole hospital after a back-up generator started working immediately, a spokesman said.
Four major electricity circuits have been badly damaged, EDF Energy said.
A statement from the company said: "The fire occurred on a cable bridge which carries the cables across a waterway and there appears to be evidence of vandalism.
"Locks have been broken on the security gates at both ends of the bridge.
"Four major electricity circuits have been badly damaged by the fire, with damage also to one of the concrete bridge supports."
EDF Energy said about 100 people were working at the site and it said the work was "complex and difficult" as the bridge needed to be safe before the engineers could attempt repairing the cables.
"We expect this work to take at least 24 to 36 hours, maybe longer," the company's statement said.
Workers would work on the repairs through the night, it added.