Power suppliers said areas of London, the South East, Midlands and South West are without electricity.
A large power cut has caused "apocalyptic" rush-hour scenes across England and Wales, with traffic lights down and trains coming to a standstill.
Strike-hit South Western Railway has not been affected, but reported numerous incidents such as tress having fallen on lines and an "operational incident" affecting services to Exeter.
There had been "issues with two power generators causing loss of power in selected United Kingdom areas", a spokeswoman for National Grid said. Social media users said that they were being asked to leave the station.
Scott McKenzie, 31, from Cardiff, said "various alarms were going off" at Newcastle Airport.
'We're aware of a power cut affecting a large area of London and South East, ' they said.
"We were literally plunged into darkness and people were using their phones as torches to see and get around", he added.
Meanwhile, Western Power Distribution (WMD) also tweeted: "We are now experiencing a major incident on the national electricity infrastructure".
Harriet Jackson, 26, said there was an "apocalyptic" scene on Northcote Road, in Battersea, when traffic lights cut out and cars were not stopping.
National Grid owns the electricity and gas transmission system in England and Wales with day to day responsibility of balancing supply and demand.
The power cuts across the United Kingdom caused pandemonium as commuters attempted to get home for the weekend.
Passengers have been warned to expect delays.
Many Thameslink trains are also "at a stand" between London and Bedford.
A Transport for London spokeswoman said some traffic lights are "not working" but the scale of the problem is not yet known.
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