ATLANTA, Georgia - The world's busiest airport said minutes after midnight Monday that power had been fully restored, some 11 hours after Hartsfield-Jackson Atlanta International was hit with a blackout. The outage grounded more than 1,000 flights and stranded thousands of passengers just days before the start of the Christmas travel rush.

The airport made the announcement on its Twitter page:

Georgia Power had said in a statement that a fire caused extensive damage in an underground electrical facility, impacting substations serving the airport. The cause of the fire wasn't immediately known.

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.

But one passenger described the situation in one terminal as "big chaos."

The outage impacted a number of flights in and out of Tulsa.  

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."

Adding to the nightmare are what some passengers said was a lack of information from airport officials and help from first responders to get the disabled and the elderly through the airport without the use of escalators and elevators.

"They had these elderly people, handicapped people lined up in wheelchairs, said stranded passenger Rutia Curry. "The people were helpless, they can't get down the stairs, it was just a nightmare."

Delta, with its biggest hub operation in Atlanta, was hardest hit. By evening, Delta had already cancelled almost 900 Sunday flights and another 300 on Monday, nearly all of them in Atlanta, according to tracking service FlightAware.com.

Delta customers flying to or from Atlanta can make a one-time change to travel plans without incurring a $200 change fee. The airline also encouraged travelers not to pick up their bags Monday because of anticipated congestion at the airport.

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.

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.

The Associated Press contributed to this report.