Thousands of homes and businesses in Bermondsey, Elephant and Castle and Kennington were without electricity on Monday afternoon.
EDF Energy says the power cut is due to a fault with an underground cable.
Traffic lights were affected in some areas, with police directing traffic at Vauxhall Cross. Vauxhall underground station was closed due to the power failure.
A spokeswoman for EDF Energy told us: "Power supplies to several thousand customers in the SE1, SE11 and SE17 areas of London were interrupted at 3.15pm after an underground cable fault."
"EDF Energy Networks engineers are working to restore power as quickly as
possible. We apologise for any inconvenience this incident is causing."
UPDATE: By 6pm supplies had been restored to all but 500 homes. EDF say that work to reconnect the remaining customers will continue through the evening.
UPDATE 8pm: Power has been restored to nearly every affected premises.