London, the south east and north west of England and Scotland were impacted although there was no immediate word on the cause of the problem. "The issue is now resolved and the system has returned to normal".
UK Power Networks said: "We're aware of a power cut affecting large parts of London and the South East".
Large parts of the United Kingdom are without electricity following a large scale power cut.
Western Power Distribution which serves the Midlands, South West and Wales said there had been a "major incident" but all affected customers now had power restored.
A Network Rail spokeswoman said a power surge on the National Grid meant power was lost to all signalling over a wide area, including Newport, Gloucester, Ashford, Bristol, Eastbourne, Hastings, Three Bridges and Exeter.
"All the traffic lights were down, but there were no police present, which meant it was risky to cross - cars weren't stopping either".
Traffic lights in the capital are also experiencing some problems and the Victoria Line on the London Underground is suspended, Transport for London confirmed.
"All trains were stopped while our back-up signalling system started up", she said. Many people reported that the outage lasted just a few minutes, but the impact on travelers was severe.
Members of the public posted images on social media, which showed the tube network in the darkness and people having to use their mobile phones as torches.
Rail services across the country were canceled or delayed during the Friday evening rush hour, and commuter trains powered by overhead wires ground to a halt in mid-journey.
It said on Twitter: "We are now experiencing a major incident on the national electricity infrastructure".