Why Did the Lights Go Out in Spain and Portugal? Here’s What We Know.

A widespread power outage upended life for millions across Spain and Portugal — and, briefly, parts of France — on Monday.

Traffic lights went dark, trains stopped, businesses closed, and long lines formed at A.T.M.s and grocery stores, where credit card readers stopped working.

The outage lasted nearly 18 hours in some areas. But as some of Spain’s top officials convened on Wednesday morning to discuss the blackout, the cause was still under investigation.

Power has mostly been restored.

Prime Minister Pedro Sánchez of Spain said the country had recovered more than 95 percent of the total power supply by 6:30 a.m. on Tuesday. The country’s Interior Ministry on Tuesday night deactivated most emergency declarations that had been activated during the blackout.

In Portugal, the electricity and gas supplier REN said power had been restored to every substation in the country’s grid and that everything was “100 percent operational.”

The electrical grid operator in France, where the outage briefly affected households in the Basque region, said all power had been restored.

The cause remains under investigation.

Spain’s National Security Council was convening on Wednesday morning to discuss the blackout.

On Tuesday, Eduardo Prieto, director of services for Spain’s national power company, Red Eléctrica, said that there were no “definitive conclusions” about the reasons for the outage, although he and other officials ruled out a cyberattack.

Mr. Sánchez said that a committee of technical experts from the European Commission would investigate the causes. He said he expected answers within “hours or days,” although others said it could take weeks to conduct a thorough analysis.

Mr. Sánchez also said in a news conference on Tuesday that the National Cryptologic Center, a department of Spain’s national intelligence agency in charge of cyberthreats, was reviewing “the risks arising from this electrical emergency.”

Some initial facts about the source of the blackout have emerged. Around noon on Monday, a high-voltage connection between France and Spain was interrupted, according to Kristian Ruby, secretary general of Eurelectric, a trade body that represents the European electricity industry. The power outage occurred just over 30 minutes later.

While that interruption would have been expected to be disruptive, it would not normally lead to a “system collapse” like the one that occurred on Monday, Mr. Ruby said. Some other complication would typically need to happen, “like a sudden outage at a power plant, a sudden development on the demand side,” he said. “Then you can have an incident like this.”

Portugal’s national energy supplier, E-Redes, cited an unspecified “problem in the European electricity grid.”

Hospitals, banks and travel were disrupted.

During the outage, there were widespread problems connecting to the internet and to phone networks across Spain and Portugal. Mr. Sánchez urged people to make only brief calls on their cellphones.

Hospitals in Spain were forced to run on generators. Portuguese banks and schools closed. Spain’s national rail company said that trains had stopped operating at all stations. Subways in several cities, including Valencia and Madrid, were halted. The Madrid Open tennis tournament was suspended. And people crammed into stores to buy food and other essentials as clerks used pen and paper to record cash-only transactions.

Diana Alfia, an employee at a hostel in Lisbon, said that some people had gone to the beach since there was not much else to do, and some tourists walked miles from the airport into the city because Uber and public transportation were not available.

Power went out amid high temperatures.

According to Spain’s meteorological agency, AEMET, temperatures across the country were between 60 and 70 degrees Fahrenheit around the time of the power outage. By the end of the day, highs had climbed to between 82 and 87 degrees.

That was higher than usual: In April, temperatures in Spain average around 64 degrees in central and northern regions and about 70 degrees in the south.

When asked whether the outage was connected to the heat, Bruno Silva, a spokesman for REN, was skeptical. “No, I hope not,” he said.

Other European outages have halted daily life.

More than 50 million people in Italy were left in the dark for almost a full day in 2003 after a line between Switzerland and Italy was overloaded. It was considered the worst day of blackouts in the country since World War II.

In 2006, 10 million people in Germany were briefly without power after the northwestern part of the country’s power grid became overloaded.

And last year, much of the Balkans was without power for several hours during a heat wave in which temperatures soared to 40 degrees Celsius, or over 100 degrees Fahrenheit.

The power grids in Europe are interconnected, and an overload or issue in one area can spread to another country.

Nazaneen Ghaffar, Mike Ives and John Yoon contributed reporting.