A massive power cut hit large parts of Spain and Portugal on Monday, causing widespread chaos and disruption. The outage left millions of people without electricity, while internet and phone networks were also down.

Spanish power distributor Red Eléctrica said that restoring power fully to the country and neighboring Portugal could take 6-10 hours.

The cause of the power outage is still unclear, although Portuguese Prime Minister Luis Montenegro says there’s “no indication” of a cyberattack.