With electricity out for about 11 hours, some passengers were unable to disembark from their planes after landing...
With electricity out for about 11 hours, some passengers were unable to disembark from their planes after landing, while those already inside the terminal crowded the darkened concourses where even some water fountains had reportedly stopped working.
"Still on plane for going on 5th hour," tweeted Anthony Foxx, former transportation secretary under president Barack Obama.
"Whatever the cause, it feels like this one was compounded by confusion and poor communication."
US carrier Delta, whose largest hub is in the southern city, said it expected 300 more flight cancellations on Monday, mostly morning arrivals, "to give the operation there an opportunity to more quickly return to normal."
Delta said it had to scrap about 900 flights on Sunday but expected its Atlanta schedule would return to normal by Monday afternoon.
Another carrier, Southwest, said it planned on operating a regular Atlanta schedule on Monday after it, and all other carriers using the airport, were affected by the power cut.
Flightview.com, which tracks air travel data, listed hundreds of flights as delayed, canceled or diverted during the outage, but on Monday it showed an increasing number of scheduled departures as well as arrivals.
"Power has been restored on all concourses," the Hartsfield-Jackson Atlanta International Airport said on its Twitter account, about 11 hours after first reporting the electrical disruption.
Giving an idea of how many people were affected, the airport added: "5,000+ meals are being delivered to passengers."
The Georgia Power company, in a statement, said "power had been restored for all essential airport activities including all concourses and flight operations."
It described the outage as "very rare." Although the exact cause is not yet known, Georgia Power said preliminary investigation suggested that a fire damaged cables in the backup power system.
"No personnel or passengers were in danger at any time," the company said.
Atlanta Mayor Kasim Reed issued an apology "to the thousands of passengers whose day has been disrupted," The Atlanta Journal Constitution reported on its website.
Photos and videos from the airport showed people walking or sitting in crowded airport corridors, with windows only partially cutting through the gloom.
Numerous airplanes could be seen parked on the airport tarmac.
A woman said she was waiting in the security line when "they turned off the lights -- a few times they went off, and then finally they shut off completely."
One passenger, Mike Vizdos, told AFP that he and others got trapped on their aircraft because the power went out just as it reached the terminal gate after arriving from Costa Rica.
"Sat on the plane for six hours and then cleared customs and immigration," Vizdos said, still hoping to reach his home in Richmond, Virginia the following day.
"There are thousands of stories," he said.
The airport earlier announced that it "sustained a power outage shortly after 1:00 pm today (Sunday)."
That led the Federal Aviation Administration to place a "ground stop" on flights inbound for Atlanta, meaning they were held at the airports from which they were to depart.
"The FAA Tower can operate normally, however, departures are delayed because airport equipment in the terminals is not working," it said.
Those stranded were not amused.
"I haven't eaten since 8AM central," one person tweeted.
Others said they were waiting on the tarmac: "Atlanta airport doesn't have power. I'm trapped on the runway, I'm hungry, and slightly buzzed..... Send help!!!"
Some said people were not being permitted to leave the airport, including a mother who tweeted: "My kid is trapped with the others at the Atlanta airport. No one is allowed to leave."