A sudden power failure 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. Shortly after midnight early Monday, authorities tweeted that electricity had been restored on all concourses at Hartfield-Jackson Atlanta International Airport.

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

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."

Delta Air Lines 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, the airline had already canceled almost 800 Sunday flights and 250 more 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 probably 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 during the week before Christmas.

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

There was one bit of good news, he added: Delta has more spare planes and available crews in Atlanta than anywhere else, which will help it to recover.

Although 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, spokesman Ross Feinstein said. The airline also diverted three planes that were headed to Atlanta when the failure occurred, 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.

By evening, power had been restored to at least one concourse. On its Twitter page Sunday night the airport tweeted, "Power on Concourse F is back ON! We are working with great urgency w/ @GeorgiaPower to restore power throughout rest of airport." Later, it tweeted: "Power is back ON in Atrium and Concourses T, A and B!"

A tweet update at 12:06 a.m. said, "Power has been restored on all concourses. 5,000+ meals are being delivered to passengers. Trains will be operational soon."

Officer Lisa Bender of the Atlanta Police Department said officers were at the airport to help control crowds and manage traffic around the airport.

At Southwest Airlines, about 70 Atlanta departures out of 120 scheduled for Sunday were canceled, an airline spokesman said in an email. United Airlines and JetBlue Airways also reported delays or cancellations.

Mozell Smith, 58, of Atlanta arrived at the airport hours after the electricity went out. He was headed to Las Vegas with a sister and a friend.

"This is terrible. I wish someone would've given us a heads-up before we got to the airport," he said. "I wish there would have been better communication."

American Airlines reported only a handful of diversions and cancellations because the carrier does not use Atlanta as a hub, airline spokeswoman Alexis Aran Coello said.

Hartsfield-Jackson, which serves 104 million passengers a year, is the world's busiest airport, a distinction it has held since 1998.

The airport serves an average of 275,000 passengers daily, according to its website. Nearly 2,500 planes arrive and depart each day.

— Associated Press