A widespread power outage hit Spain and Portugal, affecting millions of people who live across Europe's Iberian Peninsula.

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.

Power had almost fully returned to Spain early Tuesday morning as many questions remained about what caused one of Europe's most severe blackouts across Spain and Portugal.