Major parts of England were left in the dark as there was sudden power cut which is popularly known as 'Dumsor' in Ghana.
The National Grid Electricity System Operator said issues with two generators caused a loss of power, but that it was "resolved".
The company said: "We experienced issues with two power generators causing loss of power in selected UK areas.
"The issue is now resolved and the system has returned to normal. Anyone continuing to experience a local issue should contact their local Distribution Network Operator for assistance."
The problem was initially thought to affect London and the South East but also impacted swathes of the Midlands, and the North West.
Ipswich hospital was hit, and a spokesman told Sky News: "There was a national power outage and Ipswich Hospital was without power for about 30 minutes, all of our teams kept all of our patients safe, and we are now back up and running."
Western Power said 500,000 people were affected in its distribution area. They described it as a "major incident" and said all customers had power restored by 6pm.
Northern Powergrid said 110,000 customers lost power on Friday evening. The company supplies 3.9m homes in the North East as well as Yorkshire and northern Lincolnshire.
Those who were affected lost power between 5.10pm and 6pm.
Electricity North West said 26,000 people were without power in areas spreading from Penrith in Cumbria to Stockport, Greater Manchester.
Power was out between 4.54pm and 5.17pm.
In the South East, UK Power Networks tweeted on Friday evening: "We're aware of a power cut affecting large parts of London and South East.
"We believe this is due to a failure on National Grid's network, which is affecting our customers."
About 10,000 people were initially affected in London and the South East, but the company later revised this to 300,000.
Engineers created a workaround which meant the majority of customers got power back quickly. The power was out between 4.53pm and 5.21pm.
In a tweet, the company said: "The issue was on the National Transmission Network, we believe supplies should all be back up and running."
SSE said 16,000 of its customers were affected, with power going down between 4.56pm and 5.08pm.
Train services in and out of London, including Thameslink, Southern and Gatwick Express are facing delays and cancellations because of the disruption.
London North Eastern Railway (LNER) said a number of its trains had broken down between London and Stevenage, suspected to be because of the electrical supply problem.
LNER also told customers not to travel from Kings Cross and said any tickets for today would be valid tomorrow.
The north London station was evacuated.
The Metro in Newcastle was affected, with delays carrying on into the evening. Those travelling to the airport were advised to allow more time to make their journey.
A Network Rail spokesperson said: "There was a power cut on the national grid this evening which means we lost power to all our signalling over a wide area, including the Newport, Gloucester, Ashford, Bristol, Eastbourne, Hastings, Three Bridges and Exeter areas. All trains were stopped while our back-up signalling system started up.
"While our signalling system has come back online and trains are running, there will be delays to journeys in these areas and stations will be busier. Please check before you travel and thank you for your patience."
A British Transport Police (BTP) spokesman said: "We currently have BTP officers assisting at several stations owing to current disruption caused by power issues."
Traffic lights in London have also experienced problems and the Victoria Line on the London Underground was down for 40 minutes, Transport for London confirmed.
The line is now running with severe delays but all other lines are operating a good service.
TfL warned peopled to be cautious while driving on roads in the capital, but said they were not aware of any major congestion after the outage.
Rail operator Thameslink said many of its trains were "at a stand".
A tweet said: "The power network has failed in the large parts of London and the South East. This has prevented our trains between Farringdon and Bedford from being able to take power and as a result, most of our trains are currently at a stand."
Cheshire Police has said it is aware of an outage in the Ellesmere Port area, including Great Sutton and Little Sutton. It told people not to contact police, but to call their electricity providers.
In south London, customers described "apocalyptic" scenes as the power went out in Clapham Junction station.
Harriet Jackson told PA: "(I) realised that nothing was open and there was hardly any phone signal.
"All the traffic lights were down, but there were no police present, which meant it was dangerous to cross - cars weren't stopping either.
"It was like witnessing something out of an apocalyptic film.
"No one knew what was going on and, given it's a Friday afternoon, it's the last thing you want to encounter."
The Department for Business Energy and Industrial Strategy said: "We are in contact with National Grid and Energy Distribution Networks. The issue is now resolved and the system has returned to normal."
The Department for Transport tweeted: "Today's power outage has had knock on impacts on travel. We're working hard with @networkrail and others to ensure systems are up and running as quickly as possible so that everyone can complete their journeys safely."