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.

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.

The effects of a major power cut across Spain, Portugal and beyond are still being felt a day after the lights went out. Traffic lights switched off, trains stopped, and airports went dark in several cities, including Madrid, Barcelona, Valencia, Seville in Spain and Lisbon, Santarém and Porto in Portugal, following the outage around midday Monday.