It's unclear how widespread the outage is or what caused it.
At San Jose Mineta airport two inbound flights from Atlanta have been canceled. As of 2 p.m., one outbound flight to Atlanta was canceled and one has been delayed.
At San Francisco International Airport one flight has been canceled. Oakland International airport also has one canceled flight to Atlanta.
The FAA's Air Traffic Control System Command Center verified stated there were "80 to 100" planes" parked on taxiways.
1/2 The #FAA has put in a ground stop for flights headed to @ATLairport due to a power outage affecting the airport terminals. The FAA Tower can operate normally, however, departures are delayed because airport equipment in the terminals is not working.— The FAA (@FAANews) December 17, 2017
2/2 A ground stop means that flights headed to Atlanta are held on the ground at their departure airport.— The FAA (@FAANews) December 17, 2017
We will update this statement when we get new information.
Multiple airlines, including Southwest and Delta Airlines, told ABC News they were beginning to deplane passengers by sliding them down airstairs.
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."
Officer Lisa Bender of the Atlanta Police Department said officers were at the airport to help with crowd control and managing traffic around the airport.
Delta Air Lines, which has its headquarters at the airport, said more than 450 mainline and regional flights were cancelled. The airline encouraged passengers to check on the status of flights via the Fly Delta mobile app or delta.com before heading to the airport.
At Southwest Airlines, about 70 Atlanta departures out of 120 scheduled for Sunday were cancelled, an airline spokesman said in an email.
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.
"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."
Mozell Smith, 68, of Atlanta arrived at the airport hours after the electricity went off. 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.
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 & ABC News contributed to this report.