A massive power outage hit Spain and Portugal on Monday, shutting down trains, disrupting air travel and leaving large parts of the countries — including the capital cities of Madrid and Lisbon — in the dark. Power in France’s Basque region was also briefly shut down.

It’s unclear what caused the outage, which happened just after noon local time, or exactly how many people were affected. So far, officials have said there is no evidence of a cyberattack.

“Grid operators in both countries are working on finding the cause, and on restoring the electricity supply,” António Costa, president of the European Council, wrote in a post on X. “At this point, there are no indications of any cyberattack.”

Long lines quickly began forming at bus terminals and ATMs, at least the ones that were working. People bought groceries in darkened stores, using their cellphones for light.

Matches at the Madrid Open professional tennis tournament were canceled for the day. Traffic in other parts of the city came to a standstill.

The Spanish traffic authority urged people to avoid driving as the outage knocked out traffic signals across the country.

“I ask all residents of Madrid to keep their movements to an absolute minimum and, if at all possible, to remain where they are,” Madrid Mayor José Luis Martínez-Almeida said in a video posted to X. “We want to keep all roads clear.”

Spain’s national power company, Red Eléctrica, said getting its power grid back online could take up to 10 hours. The company said that power was gradually being restored in northern areas.