VIDEO-Delta Warns of Chaos After Power Outage, Worldwide System Failure - NBC News

A handwritten Delta boarding pass, issued because the airline's computer system is down, given to passenger Rob Stephenson, 42, of Ann Arbor, Michigan at London's Heathrow Airport. Rob Stephenson

Luciano Resende, 40, had been waiting for at least two hours at Heathrow as of 4:40 a.m. ET. He was attempting to make the trip back home to San Francisco via Seattle.

He said airline employees started to manually check-in customers for their their flights but progress had been slow. "I guess it has been a long time since they used the manual process," Resende told NBC News.

At Tokyo's Narita airport, passenger Brett LaBare endured a delay of more than five hours until his flight to LAX was eventually canceled because the crew exceeded their safety-related working hours limit.

"A Delta ground stop has been lifted and limited departures are resuming following a power outage in Atlanta that impacted Delta computer systems and operations worldwide," the airline said in an 8:40 a.m. ET update. "Cancellations and delays continue."

It added: "Customers heading to the airport should expect delays and cancellations. There may also be some lag time in the display of accurate flight status at delta.com, the Fly Delta App and from Delta representatives on the phone and in airport."

Last month, Southwest Airlines canceled more than 2,000 flights over several days after an outage that it blamed on a faulty network router.