Thousands of homes in east London were plunged into darkness for more than five hours after a power cut.
EDF Energy said the supply went off to 6,000 properties in the Hackney area at about 1830 BST on Friday.
Motorists also faced serious disruption in Homerton and Hackney Wick as 38 sets of traffic lights went out.
Teams of engineers working at various sites discovered there was a fault at a sub-station and the power was restored at about midnight.
A company spokeswoman said: "We apologise to our customers for the inconvenience this has caused."