A sudden power outage brought the world's busiest airport to a standstill Sunday, grounding more than 1,000 flights in Atlanta just days before the start of the Christmas travel rush. Hours after the blackout began, authorities announced that electricity would be restored at the Hartfield-Jackson Atlanta International Airport by midnight.

Passengers at the airport were left in the dark when the lights went out at around 1 p.m. The outage halted all outgoing flights, and arriving planes were held on the ground at their point of departure. International flights were being diverted, officials said.

Mayor Kasim Reed tweeted Sunday night that all passengers had been safely deplaned.

The City of Atlanta said on its Twitter page that it would provide shuttle service to the Georgia Convention Center for travelers in need of a place to stay and Chick-fil-A would be provided.

Delta passenger Emilia Duca, 32, was on her way to Wisconsin from Bogota, Colombia, when she got stuck in Atlanta. She said police made passengers who were in the baggage-claim area move to a higher floor. She said restaurants and shops were closed. Vending machines weren't working.

"A lot of people are arriving, and no one is going out. No one is saying anything official. We are stuck here," she said. "It's a nightmare."

Delta, with its biggest hub operation in Atlanta, will be hardest hit. By evening, Delta had already cancelled almost 800 Sunday flights and another 250 on Monday, nearly all of them in Atlanta, according to tracking service FlightAware.com.

Robert Mann, an aviation consultant and former American Airlines executive, said it likely will be Tuesday before Delta's operations in Atlanta return to normal, and for passengers "it could be most of the week" because there aren't many open seats on other flights in the last week before Christmas.

"Tomorrow is going to be a long and difficult day for everybody," Mann said.

One bit of good news, according to Mann: Delta has more spare planes and available crews in Atlanta than anywhere else, which will help it to recover.

Still, when flights at Atlanta were grounded for most of one day last spring, it took Delta five days — and about 4,000 cancelled flights — before it fully recovered.

Like Sunday's outage, that April storm hit Delta's largest hub at a busy travel time when there weren't many empty seats to accommodate customers from cancelled flights. At the time, CEO Ed Bastian vowed that Delta would make "significant improvements" to its system for scheduling and tracking aircraft crews to recover more quickly from disruptions.

While Delta was hit hardest by the outage, other airlines also canceled flights for the rest of Sunday. American Airlines canceled 24 departures and an equal number of arrivals, said spokesman Ross Feinstein. The airline also diverted three planes that were headed to Atlanta when the outage struck, sending them instead to Dallas, Nashville and back to Philadelphia.

The FAA said it would staff the airport control tower throughout the night so that it can handle flights once they resume. The FAA said the tower could operate normally but flights were affected because airport equipment in the terminals was not working.

According to a Georgia Power statement, a fire in an underground electrical facility may have been responsible for the outage. The cause of the fire was not known.

"No personnel or passengers were in danger at any time," the statement said.

No areas outside of the airport were affected by the power loss. The utility said that there are "many redundant systems in place" to ensure the power supply to the airport and that such outages at the airport "are very rare."

That wasn't enough to comfort Jeff Smith, 46, of Pittsburgh, who ended up stuck in a plane on the tarmac for three hours after it landed.

"This is the worst experience I've ever had at an airport," he said.

Sara Melillo, who was traveling to Pittsburgh from Kenya, where she lives with her husband, Greg Presto, to spend Christmas with his family were stuck on the tarmac for six hours. The couple had made stops in Nairobi and Amsterdam and landed shortly after the lights went out in Atlanta.

She said the pilot didn't have a lot of information for the travelers but the plane had air conditioning and attendants offered water and juice a few times. She described the Delta terminal as "big chaos" with not enough customer service for the hundreds of people trying to find a flight to their next destination and a place to sleep for the night.