Tens of thousands of people have been affected by a power outage at Atlanta’s Hartsfield-Jackson international airport that led to a partial shutdown on December 17.

Atlanta’s Hartsfield-Jackson airport is the world’s busiest, handling more than 250,000 passengers and almost 2,500 flights every day.

Passengers were left in darkened terminals or on board planes.

Power is now restored at the airport, but there have been more than 1,000 flight cancelations, with hundreds more scheduled for December 18.

Many flights scheduled to arrive from other airports were being diverted elsewhere or held at their departure airport.

In a statement, the airport confirmed they had suffered a power cut shortly after 13:00 local time on December 17. Flights are expected to resume at 06:00 local time on December 18.

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Georgia Power, which supplies the airport’s electricity, said it believed a fire at an underground electrical facility was to blame for the power cut. According to officials, a piece of its switchgear could have failed and started the fire, causing cable damage.

Power was fully restored to the airport late on Sunday, December 17.

Atlanta’s mayor confirmed the fire’s cause was under investigation, and apologized to the thousands affected.

A number of major airlines, including United, Southwest and American Airlines, completely suspended their operations on December 17.

Images shared on social media showed passengers waiting in darkness. Some reported being stuck on board aircraft for six hours.

The local police department confirmed they had sent extra officers to help the airport with the situation.

About 30,000 passengers were reportedly affected by the power cut.

Atlanta is located within a two-hour flight of 80% of the US population, making the city a major port of entry into the US and a common stopover for travel within the country.