Power was gradually restored for millions on Monday following what Spanish officials called a "rare atmospheric phenomenon."

MADRID/LISBON (Reuters) -A huge power outage brought parts of Spain and Portugal to a standstill on Monday, grounding planes, halting public transport, causing panic buying and leaving the two countries scrambling to restore power to millions of homes and businesses.

A countrywide power outage impacted Spain—as well as swaths of Portugal and smaller portions of France—Monday afternoon, affecting essential services and wreaking chaos across the Iberian Peninsula. The cause of the outage is currently being investigated by Spanish and Portuguese officials and ministers.