A power outage brought the world’s busiest airport to a standstill Sunday, ruining holiday travel for thousands and threatening another day of headaches, with hundreds of flights on Monday already canceled.

Hartsfield-Jackson International Airport in Atlanta lost power shortly after 1 p.m. ET on Sunday, leaving passengers trapped inside airport terminals and some in planes on the tarmac for hours.

By late Sunday, the airport had canceled 1,173 flights and delayed 207, according to FlightAware.com. Delta Air Lines, which is headquartered in Atlanta, said it was canceling another 300 flights, mostly to Atlanta, on Monday.

Some 80 to 100 planes got stuck on the tarmac, as powerless gates became unusable. Southwest and Delta Airlines — which use the airport as their largest hubs — both told ABC News that they were beginning to deplane passengers by having them slide down emergency airstairs.

Some passengers inside the airport had to use their mobile-phone flashlights to get around, while passengers on the disabled SkyTrain rail system had to be rescued by firefighters. There have been no reported injuries so far.

Police began evacuating people from the crowded airport, and later the city opened the Georgia International Convention Center and offered free shuttles for stranded passengers who needed a place to stay overnight.

Just before midnight Sunday, around 11 hours after the initial outage, all power had been fully restored to the airport.

Hartsfield-Jackson is one of the busiest airports in the world, serving an average of 2,500 flights and 275,000 passengers on any given day. Mayor Reed estimated that 30,000 people were affected by the outage on Sunday, and the airport’s problems will undoubtedly cause ripple effects throughout the air-transit system.