More than 1,500 flights into, within or out of the US were canceled before 11 a.m. Sunday, according to the tracking website FlightAware.com.
In Chicago, O'Hare airport had 127 delayed flights with 76 cancellations and Midway airport was reporting 39 delays and two cancellations, according to FlightAware.com.
Nearly half of Sunday's cancelations were from Delta Air Lines, with at least 500 flights affected.
"Additional cancellations are expected as some of Delta's technology continues to recover from Friday morning's vendor-caused issue," the airline said in a statement Saturday morning.
On Saturday, 2,136 US flights were canceled, and more than 21,300 flights were delayed, according to FlightAware.
The issue extended beyond airports, with businesses, government agencies, health and emergency services, banks, schools and universities around the world grinding to a halt or seeing services disrupted due to a flawed software update for Microsoft Windows operating systems issued by the cybersecurity firm CrowdStrike, experts told CNN.
The outage "has basically turned computers into bricks around the world," Glenn Gerstell, former general counsel of the US National Security Agency, told CNN on Saturday.
"This is probably going to be the biggest single computer incident in terms of overall effect," Gerstell said. "Maybe not the number of computers, but the impact on people's lives."
CrowdStrike CEO George Kurtz apologized to customers and said a fix has been deployed, but experts say getting systems back in order will be a lengthy process.
The outage affected an estimated 8.5 million Windows devices, less than 1% of all Windows machines, according to a Saturday blog post from Microsoft. "While the percentage was small, the broad economic and societal impacts reflect the use of CrowdStrike by enterprises that run many critical services," Microsoft said.
Major airlines have said services are being restored, but that there could be more delays and disruptions.
A majority of United Airlines systems have recovered from Friday's outage, the airline said in a statement. More than 400 United flights were canceled Saturday, and over 200 flights were canceled Sunday, according to FlightAware.com.
"While most of our systems have recovered from the worldwide third-party software outage, we may continue to experience some disruption to our operation, including flight delays and cancellations," United said.
Delta has paused unaccompanied minor travel until Monday due to the outage, the airline announced over the weekend. Unaccompanied children who are already booked on a Delta flight will not be able to travel, and the airline has asked that no new flights be booked for them.
Also impacted was American Airlines, which said in a statement Friday that "we were able to safely reestablish our operation" and it had "issued a travel waiver for our customers impacted by the vendor technology issue earlier this morning." Allegiant Air said in a statement Saturday while "normal operations have resumed" after the outage, they're processing a backlog of customer messages and troubleshooting their programs and platforms.
Even with the flawed computer update rolled back, it's not a quick fix for airlines, which have computers at thousands of gates that will need to be individually manually rebooted, David Kennedy, cofounder of cybersecurity company Binary Defense, told CNN on Saturday.
"It's not just as simple as rebooting. There's a lot more steps and complexities in this that are involved," Kennedy said. "There's just not enough people at those airports, at those locations to go and do it."
The US Department of Transportation on Friday said it determined the flight delays and cancellations resulting from the system outage were "controllable," meaning they're "attributable to the airline." In such cases, the airlines "must adhere to their customer service commitments."
Transportation Secretary Pete Buttigieg in a social media post Saturday said he received reports of some airlines only offering flight credits to passengers for canceled flights.
"Let me be clear - you are entitled to get your money back promptly if your flight is canceled and you don't take a rebooking," Buttigieg said.
Travelers upset as summer travel plans are thwarted
Frustrated passengers lined up at airports backed up with flight cancellations and delays, some missing life events like weddings, funerals and birthdays.
At Atlanta's Hartsfield-Jackson International Airport, long lines of passengers waited for assistance Saturday, with many flights still canceled or delayed.
Delta passenger Catalina Villareal described the scene: "Mayhem. Chaos. Frustration. Hunger."
Villareal expressed frustration with what she says is a lack of information from airlines.
"I have had three cancellations. I've checked in three times for all of the flights. And now I have two bags some place ... in the airport, and nobody knows how long they'll take to come in," Villareal said. "So, we've been told just to come tomorrow or Monday."
Thousands of people face the same uncertainty as they cram together in airports across the country, waiting for answers.
"I was supposed to be in California for my mom's wedding," Richard Whitfield of Pasco County, Florida, told CNN on Saturday. Whitfield and his partner, Jonathan Shade, left Tampa on Thursday and missed their connecting flight in Atlanta due to poor weather conditions, delaying their landing and forcing the plane to refuel in Tallahassee.
After their rescheduled flight on Friday was delayed many times, the couple decided to cancel the trip and just head home. But with no available flights back to Tampa on Friday night, they spent their second night in an airport hotel. They were unable to get a voucher from Delta for either of their stays.
"(Richard has) been on hold for 24 hours," Shade told CNN. "When he eventually got his number in line, it was 2,001."
Two hours later, Richard's place in Delta Air Lines' virtual customer service queue was 2,300 in line, Shade said.
Whitfield told CNN the whole ordeal has had an impact on him.
"For me, it's been the domino effect that it has on humanity and everything that we need to survive: food, sleep, or water, housing," he said.
After spending 48 hours in Atlanta, they found a Saturday evening flight back to Tampa they say they can only hope is not delayed or canceled. For now, there is nothing the couple can do but wait and "get a good stiff drink," Shade and Whitfield said.
Another traveler at Boston Logan International Airport was trying to get to Fort Lauderdale for her father's 96th Birthday.
"My flight this morning was canceled. I was supposed to fly to Fort Lauderdale, so they rebooked to West Palm Beach and I got here to the airport, it was canceled. They didn't tell me - I had no notification, nothing," Charlotte Yeh told CNN affiliate WFXT on Saturday.
Some passengers at the Boston airport were upset that their summer trips and plans were foiled.
"We've had this trip booked to Las Vegas for some time now," Marc Forbes told WFXT. "Straight up cancellation with the next possible flight being Monday at 6 p.m. and we were only going to be in Las Vegas for four nights so that trip is going to have to be rebooked."
Carol Edwards said her flights on Friday and Saturday were both canceled, and the next available flight isn't until Monday.
"We have a lot of plans you know visit family, eat, see friends, party - you know, everything - so there's nothing we can do," she told WFXT.
ABC7 Chicago contributed to this report.
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