Commuters had to use the torches on their phones as they walked in complete darkness at Clapham Junction during a power cut.
Traffic lights stopped working, trains were cancelled, and stations were evacuated after a technical fault at two power generators run by National Grid triggered a 'major incident'.
On Friday evening, there were reports that the problems may have been caused by issues at a gas-fired power station - and at a wind farm off the coast of Yorkshire.
'What happened is a major offshore wind generation site and a gas turbine failed at the same time,' Devrim Celal, of Upside Energy in London, a contractor with National Grid, was reported saying. 'There was a significant shortage of generation, and that sudden drop created ripple effects across the country.'
Professor Tim Green, co-director of the Energy Futures Laboratory, Imperial College London, believes the two generators disconnected were at Little Barford in Bedfordshire and Hornsea in Yorkshire.
He said: 'The first generator to disconnect was a gas fired plant at Little Barford at 16:58. Two minutes later Hornsea Offshore wind farm seems to have disconnected. This might be linked to disturbance caused by first generator failing; might not. We will need to wait for National Grid's full technical investigation to get to bottom of that.'
Commuters described 'apocalyptic' scenes as they tried to make their way home during the evening rush-hour.
And nearly a million customers were left without electricity following the outage at around 5pm. Blackouts were reported in London and the South East, as well as the Midlands, South West, North East and the North West. The National Grid Electricity Operator took around two hours to resolve the issue.
The events immediately sparked speculation over the cause. Last night, the National Cyber Security Centre said there was, as yet, no evidence of hackers assaulting the power network.
Comment: As we've clearly seen in the past, 'no evidence' is rarely a reason not to blame Russia. Give it time.
Embarrassingly, Chancellor Sajid Javid visited National Grid bosses and trainees at its Eakring Training Centre, just hours before the power cut. The mayhem last night was at its worst on the transport network. Commuters travelling on the London underground and at Clapham Junction were plunged into darkness.
King's Cross in London, one of Britain's busiest stations, had to be evacuated and thousands of commuters were trapped on trains which were left stranded. All services were cancelled between London and Bedford, Cambridge and Peterborough. At Newcastle airport, travellers looked on bemused as information screens went blank.
Hundreds of thousands of households and businesses, from the South East to the Lake District, also lost power for up to an hour from around 5pm.
One energy distributor, Northern Powergrid, said 110,000 of its customers in Yorkshire and the North East lost power. A spokesman said: 'In all my years working here I have never seen [a power cut] so widespread and affecting so many areas of the country before.'
A Government spokesman said: 'We are in contact with National Grid. The issue is now resolved.'
It came as:
- Passengers reportedly collapsed due to heat exhaustion after a Tube train got stuck near Oxford Circus
- Generators in Little Barford in Bedfordshire and Hornsea in Yorkshire were blamed for the major outage
- Ofgem demanded an urgent report from the National Grid following power cuts which lasted two hours
- Ipswich Hospital lost power for half an hour after its back-up generator failed with operations cancelled
- Reports of people 'thrown off' an escalator in Bradford due to the sudden loss of electricity in the area
- Outage was described as 'apocalyptic' by witness to traffic light outage in Clapham, south-west London
'However, Ofgem understands the frustration this power cut has caused consumers. Ofgem has asked for an urgent detailed report from National Grid so we can understand what went wrong and decide what further steps need to be taken. This could include enforcement action.'
In London, commuters were plunged into darkness on the Victoria Line while traffic lights in Battersea lost power. All services out of London King's Cross were cancelled, and Thameslink trains were held 'at a stand' as British Transport Police confirmed they were assisting station staff and crowds.
Passengers reportedly collapsed due to heat exhaustion after a Tube train got stuck between Oxford Circus and Green Park for 25 minutes.
Passengers were also left waiting on planes in Luton and Newcastle, with the former airport also affected by flooding this afternoon. In Ipswich, a hospital was left without power - as was St Mary's in Paddington.
Elsewhere, in Derbyshire, homes were left entirely without power with one resident telling MailOnline of burglar alarms going off and police sirens in the area. Another said they feared missing their honeymoon due to the travel chaos.
Concerned residents reported power outages at their homes. One in Derbyshire told MailOnline: 'Our whole estate had all the burglar alarms going off, and there were a few police sirens.'
In Bradford, there even reports that some people had been 'thrown off' and escalator which suddenly stopped due to an outage.
Large swathes of the country were left affected by the outage, which included London and the south-east of England. Also affected was the Midlands and parts of south-west England.
'Another train has been told to get on our train as well so it is overfilled, hot and cramped.'
Harriet Jackson described an 'apocalyptic' scene when she witnessed the power outage causing traffic lights to cut out on Northcote Road in Battersea, London, after leaving Clapham Junction train station at around 5pm.
'(I) realised that nothing was open and there was hardly any phone signal,' the 26-year-old said.
'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.'
And at a Sainbury's in London, people appeared to make off with free shopping, with an eye-witness saying: 'No one knew what to do. The staff just asked everyone to abandon their trolleys and leave.
'But with no security cameras or alarms, people were just leaving with all their shopping. Someone walked out with a hoover.'
In a tweet after the outage, the National Grid said: 'We had an unexpected and unusual event, the loss of two generators that connect to the GB transmission system, which led to a fall in the frequency of the electricity system.
'Even though these events are outside of our control, we have plans in place to respond and the system operated as planned by disconnecting an isolated portion of electricity demand.
'We appreciate the inconvenience caused but this action allows the system to protect itself and limit the fall in frequency allowing for power to be quickly restored.'
Julian Leslie, head of national control for the National Grid, said: 'The system need to protect itself and [...] as the frequency was falling, the system could see this and unfortunately the way the system protects itself is by losing some demand.'
Elisabeth Braw, of the Royal United Services Institute think tank, said the UK should stress-test its reliance on electricity. 'Whatever the cause is, the effect is essentially the same,' she said. 'Whether it's nature or a hostile attack, it causes disruption.'
A back-up generator failed at Ipswich Hospital after the power cut, a hospital spokeswoman said.
The generator which was supposed to supply power to outpatient areas of the hospital did not work as expected.
'There were some issues with regard to our outpatient areas and the generator that provides cover (to them),' said a spokeswoman for East Suffolk and North Essex NHS Foundation Trust.
She said other generators 'kicked in as was required' and 'patients were kept safe and cared for throughout'.
Passengers at King's Cross Station in London were among those affected, with the station evacuated and commuters not allowed to enter through the ticket barriers.
One of those affected, Richard North, said: 'Would be interesting to know who will compensate the £120 taxi or the fines for collecting children three hours late from two separate childcare providers.
'Two children who were meant to be in bed an hour ago and we're still in a taxi.'
UK Power Networks, Western Power Distribution in the Midlands and Northern Powergrid all reported outages - due to a failure on National Grid's network.
Around 300,000 UK Power Networks customers were affected in London and the South East, a spokeswoman said and Western Power Distribution said around 500,000 people were affected in the Midlands, South West and Wales, with power restored to them all shortly after 6pm.
A spokeswoman for Northern Powergrid, which serves Yorkshire and the North East, said 110,000 of its customers lost power, while at least 26,000 people were without power in the North West of England, Electricity North West said.
Police have been drafted in to deal with the power cuts, which have stopped traffic lights in some areas of the country.
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.'
Western Power Distribution shared a similar message, and said they are in the process of restoring power to customers.
Commenting on the rail disruption, a Network Rail spokeswoman said: 'There was a power surge 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.'
Police in the capital could also be forced to man busy junctions after traffic lights failed.
A Transport for London spokeswoman said some traffic lights are 'not working' but the scale of the problem is not yet known.
Police officers could be called in to 'manage the busy junctions, to physically manage them themselves', she said.
'We're just assessing how many traffic signals are out.'
Elsewhere in the country, Cheshire Police said they were aware of a power cut in the Ellesmere Port area.
The drop in power also affected travel.
Train services in and out of London, including Thameslink, Southern and Gatwick Express were facing delays and cancellations.
Traffic lights in the capital were also experiencing some problems and the Victoria Line on the London Underground is suspended, Transport for London confirmed.
They tweeted on Friday evening warning people to be cautious when using the roads.
The Transport for London traffic news account said some traffic lights were out as a result of powercuts.
It tweeted: 'Due to a large scale National Grid failure there is a power cut in the London and South East areas, meaning that some traffic lights are down. Please be very cautious on the roads!'
British Transport Police officers are assisting at train stations after power cuts caused widespread disruption.
The force tweeted: 'We currently have BTP officers assisting at several stations owing to current disruption caused by power issues.'
Elsewhere in the country, Cheshire Police tweeted: 'We're aware of a power outage in the Ellesmere Port area, including in Great Sutton and Little Sutton. Our call handlers are unable to assist, please contact your electricity provider for advice.'
Scott McKenzie was travelling through Newcastle Airport when the power cut out for around 15 minutes, in what he was told by officials was 'due to a wider problem in the region'.
'It was a bit worrying to start - more so because various alarms were going off and staff clearly didn't quite know what was going on either,' the 31-year-old from Cardiff said.
'Some of the spaces in the airport have little daylight - we were literally plunged into darkness and people were using their phones as torches to see and get around.'
Mr McKenzie said the problem at the airport 'seems to have been resolved' and Newcastle Airport said it is aware of the power issues but flights have not been disrupted.
A Govia Thameslink Railway spokeswoman said: 'A wide scale failure of the power grid has brought many of our trains that use the overhead wires to a stand.
'This is causing significant disruption to Thameslink and Great Northern trains.
'Services may be cancelled, delayed or revised until the end of the day.
'We have put in place alternative travel advice which is available on our website and National Rail Enquiries and on social media, and would urge passengers to check before they travel.
'We are doing everything we can to keep passengers informed and get them moving again.'
Southern and Gatwick Express services were running.
London North Eastern Railway (LNER) said in a statement: 'A number of trains have broken down between London and Stevenage. We suspect this has been caused by an electrical supply problem.
'Grand Central, Hull Trains and LNER services to and from London Kings Cross may be cancelled or delayed by up to 60 minutes.
'We don't have an estimate yet for when the problem will be resolved, however disruption is expected to continue until at least 8pm.'
Problems with power were first detected late on Friday afternoon, when UK Power Networks, who control power lines for London and the South East, and Western Power Distribution in Midlands, the South West and Wales both confirmed widespread outages.
R.C.