CNN  — 

Spain and Portugal are reeling from a massive, unexplained power outage that knocked out traffic lights, caused chaos on roads and in airports, and prompted both countries to declare a state of emergency.

Portugal’s grid operator Redes Energéticas Nacionais (REN) said electrical supply was lost across the entire Iberian Peninsula, and in parts of France, shortly after midday. Hours later, Spain’s Prime Minister Pedro Sanchez said authorities were still not sure what caused the blackout.

The outage took out lighting and power sockets, and caused subway systems to suddenly fail. In Madrid, traffic piled up on the roads after the lights went out.

“I was driving and suddenly there was no traffic lights … It was a bit of a jungle,” Luis Ibáñez Jiménez told CNN. “I saw a massive bus coming, and I had to accelerate a lot to go past it.”

The cause of the blackout was unclear, but its impact was dramatic: transport hubs were shuttered and governments in both countries, which share a population of around 60 million people, hastily arranged emergency meetings to co-ordinate a response.

Spain’s Interior Ministry declared a state of emergency in the regions of Andalucia, Extremadura, Murcia, La Rioja and Madrid. After a late-night cabinet meeting, Portugal’s Prime Minister Luis Montenegro declared an energy crisis, with the country’s grid operator warning that fully restoring power would be a “complex operation.”

By nightfall, grid operators said the supply of energy was gradually being restored in pockets of both countries. Nearly half of Spain’s energy demand was being met by Monday evening, according to Environment Minister Sara Aagesen. Videos on social media showed people in Portugal cheering when power was restored as the night drew in.

Earlier, Madrid’s mayor José Luis Martinez Almeida asked people to minimize their movements and only call emergency services if it was truly urgent. He also called on people to clear the roads for emergency workers. Later in the day, Madrid’s emergency services provider urged the country’s government to declare a national emergency, and local leader Isabel Díaz Ayuso asked the country to deploy the army.

Antonio Costa – president of the European Council and Portugal’s former prime minister – said although the cause of the outage was not clear, there were “no indications” of a cyberattack.

Portugal’s prime minister blamed his neighboring nation for the scenes, saying his government did not yet know what caused the cut, but that it “did not originate in Portugal” and “everything indicates” that the problem started in Spain.

João Faria Conceição, head of REN, said Portugal was badly affected because it imports electricity from Spain in the morning, because Spain is one hour ahead and electricity produced by its solar plants is cheaper than producing it internally, during those hours.

“We are peripheral,” Conceição told a news conference Monday evening. While Spain received support from France and Morocco, Portugal had no country to turn to for emergency supplies of electricity.

Monday’s blackout hit a huge and busy swathe of southern Europe. Dozens of Iberian cities, like Madrid, Lisbon, Barcelona, Seville and Valencia, are major hubs for transport, finance and tourism. Two of the five busiest airports in the European Union in 2023 were Madrid’s and Barcelona’s, according to EU data.

For a few hours, modern routines were suspended: cash replaced card payments, police officers used arm signals to direct traffic, and restaurants, supermarkets and stores closed their doors. Madrid’s firefighters carried out 174 “elevator interventions” across the city on Monday, its Emergency Information Office said, and some shoppers stocked up on essentials and on canned goods.

The worst-case scenarios appear to have been averted, at least in the first hours of the blackout. Spain’s nuclear sites were declared operational and safe, while Portugal’s National Institute for Medical Emergencies said it had “activated its contingency plan,” running its telephone and IT systems through a back-up generator. Spain’s health ministry said the same process happened in hospitals there.

But travel was hit harder. Flights at major airports in the region were suddenly delayed or canceled, with travelers scrambling to adapt; online flight trackers reported that several airports saw their frequent departures suddenly halted after midday. Portugal’s flag carrier TAP Air Portugal told people not to travel to the airport until further notice.

Ellie Kenny, a holidaymaker inside Lisbon’s Humberto Delgado airport, said hundreds of people were stood in the dark in lines, with no air conditioning or running water. Shops were only accepting cash, she told CNN.

Trains were also suspended in Spain. And darkness suddenly descended in subway tunnels; video posted on social media showed blackened subway cars stuck in standstill on platforms in Madrid, where the metro was suspended and entrances to stations were taped off.

Sporting events were impacted too. Tennis fans at the Madrid Open filed out of courts after the outage caused play to be suspended.

Some parts of southern France, near the Spanish border, felt a more sporadic impact.

Emilie Grandidie, a spokeswoman for France’s electricity transmission operator RTE, told CNN there was “a small power cut” in the French Basque Country; “It lasted only a couple of minutes and was restored very quickly,” she said.

For several hours on Monday, tens of millions of people were asking each other when power would return, and why it was knocked out in the first place.

Neither question was easy to answer. But once power returns, it could still take days to untangle the damage caused by Monday’s worrying blackout.

Spain’s transportation minister said medium and long-distance trains won’t resume service until at least Tuesday, and the impact of a huge backlog in flights could stretch throughout the week.