Officials in Spain and Portugal are racing to restore electricity after a huge power outage that grounded flights, paralyzed metro systems, disrupted mobile communications and shut down ATMs, upending the lives of tens of millions of people and turning airports and train stations into campgrounds for stranded travelers.

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. Trains were cancelled and evacuated in some regions, traffic lights stopped working amid mounting delays at airports.

Power has been restored to tens of millions of people across almost all of Spain and Portugal, but disruptions to transport services were expected to stretch into a second day as questions mount over what caused a massive blackout.