Nearly 900 homes and offices around Blackfriars Road experienced a power cut on Friday morning.
Power went off at about 10.15am and was restored to many premises by about 10.35am.
Traffic lights in Blackfriars Road were also affected by the failure.
"UK Power Networks engineers worked quickly to safely restore power supplies to 869 customers in the Southwark area after an underground cable fault at 10.16am today," said a spokesman for the electricity company.
"Other customers nearby might have experienced a brief interruption of less than a minute while this problem was rectified. We apologise for any inconvenience this incident caused."
Some homes and offices were still in darkness at 1pm. UK Power Networks engineers were working at a substation in Chancel Street.