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.

A power outage caused chaos at a train station in Madrid, Spain, on Monday afternoon. Footage shows distressed passengers stranded and overcrowding the terminals on April 28. Officials said the electricity cut was part of a major grid failure that impacted large parts of Spain and Portugal.

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.