A large power outage in Spain and Portugal has cut phone service and shut down traffic lights, and power may take hours to restore.

Both the capitals of Madrid and Lisbon were affected by the power outage, and subway trains and ATM machines were also shut down, The Associated Press reported. About 50 million people live in Spain and Portugal.

Red Eléctrica, a Spanish power distributor, said it could take 6-10 hours to restore power to large parts of the country and Portugal. The company declined to speculate as to the cause of the power outage, but the Portuguese National Cybersecurity Center said there was no sign of a cyberattack causing it.

Portugese Cabinet Minister Leitão Amaro said the cause of the power outage “looks like it was a problem with the distribution network, apparently in Spain. It’s still being ascertained.”

E-Redes, a Portuguese distributor, said the outage was due to “a problem with the European electricity system.”

The Spanish Parliament closed and matches at the Madrid Open tennis tournament were suspended.