The power went out at 12.33pm local time in Spain and Portugal — and soon afterwards, life in major cities along the Iberian Peninsula came to a halt.
A state of emergency has been declared following the huge power outage, which Spanish grid operator REE said was caused when a sudden, large drop in power supply caused the grid interconnection between Spain and France to trip.
Hospitals resorted to back-up power, public transport stopped, cities were gridlocked and more than 50 million people were urged to return home.
The latest update from the Spanish grid operator is that a "return to normal will still take several hours."
Spanish Prime Minister Pedro Sánchez said the outage had caused "serious disruption" for millions and "economic losses in businesses, in companies, in industries".
The network lost 15 gigawatts of electricity generation in five seconds at around midday local time, the Spanish energy ministry said, without explaining the reason for the loss.
REE's system operations chief Eduardo Prieto told reporters the loss of power supply was beyond the extent that European systems are designed to handle and caused the Spanish and French grids to disconnect.
That in turn led to the collapse of the Spanish electricity network.
"As the result of this disconnection and the serious imbalance of band generation that is in our electrical system, the electrical system collapsed," he said in a news conference.
The ultimate reasons behind the massive loss of power were not identified by REE staff and no hypothesis was being ruled out, Spain's PM said in national address.
Here's how the power cut caused chaos in the region.
Hospitals switched to back up power
Hospitals and other emergency services in Spain and Portugal were forced to switch to generators.
The generators were used to keep critical wards going, but some other units were left without power.
Routine operations were cancelled in some hospitals and patients dependent on oxygen machines at home were also affected, El Pais reported.
Portugal's electricity distributor REN said it was prioritising supply resumption to hospitals.
Gridlocked cities and transport cancellations
Transport was heavily affected, with people travelling via car, bus, train, plane and even cable car all stuck at a standstill.
On the roads, police were conducting traffic at some major junctions because traffic lights had stopped working.
Many roads in Madrid, Barcelona, Lisbon and other affected cities were gridlocked.
There were long queues at bus stops and many people appeared to have decided to walk home, with the streets full of pedestrians.
A young girl, Marina Sierra, tried to contact her dad and improvise a route home in her Madrid suburb after her school was shut.
"The building we were in was giving off smoke, they had to evacuate us quickly," she said.
"I'm shocked because everything is totally out of control."
The entire train network came to a halt as passengers were forced to wait on platforms with no trains in operation.
Others weren't so lucky, as some trains came to a screeching halt on the line as they were moving at high speed.
Carlos Condori was on the Madrid metro when the blackout brought his journey to a shuddering halt.
"The light went out and the carriage stopped," but the train managed to crawl to the platform, the 19-year-old construction worker told AFP outside a metro station in central Madrid.
"People were stunned, because this had never happened in Spain," he added.
Panic buying at supermarkets
Businesses were taking measures "to prevent panic buying and ensure the safety of all," retail industry association EuroCommerce said.
In Portugal, many supermarkets in the capital Lisbon and the resort town of Vilamoura had shut.
In a supermarket in Madrid, people were stocking up on water, eggs and milk and some shelves were already empty as long lines formed for the tills.
"I left work and the power suddenly went and people started buying," said Joanly Perez, a 33-year-old doctor in Madrid.
"People aren't sure what might happen and are buying basic essentials, just in case."
Many stores and taxis in Madrid were no longer accepting electronic payments, and some local shops were giving credit to frequent customers.
Retailers including grocery chain Lidl and IKEA shut stores across Spain and Portugal after the outage.
Spare a thought for people who were stuck in lifts and cable cars as the power went out.
Emergency services workers carried out 286 rescue operations to free people trapped inside elevators in Madrid, the head of the regional government said.
Candles at Madrid Open
Play at the Madrid Open was cancelled, forcing 15th seed Grigor Dimitrov and his British opponent Jacob Fearnley off court as scoreboards went dark and overhead cameras lost power.
The round of 32 match at the Manolo Santana Stadium where Dimitrov led Fearnley 6-4, 5-4 had to be stopped due to a spidercam being left hanging too close to the court.
"The nationwide power cut experienced in Spain on Monday 28 April has forced the cancellation of both the day and night sessions," the Madrid Open posted on X.
Fourth seed Coco Gauff, who beat Belinda Bencic 6-4, 6-2 to reach the quarter-finals, had her post-match interview at the Arantxa Sanchez Stadium cut short with the microphone losing power.
Nuclear power reactors shut down
The operators of Spain's nuclear reactors were preparing to reconnect to the power grid as they recovered their external electricity supply, the country's nuclear safety council said.
It had said earlier that the reactors were in safe condition.
Four reactors stopped operating automatically after the outage, after which emergency generators kicked in.
Spanish oil company Moeve said it had halted operations at its oil refineries in the region as well.
Madrid, Andalusia and Extremadura asked for the central government to take over public order and other functions.
Pilar Lopez, a higher education administrator from Madrid tried to put some perspective on the situation.
"We've suffered a pandemic, I don't think this is worse," she said.
"It's like anything, you get used to it and start to think that this isn't the end of the world."
ABC/wires
