The FAA tweeted at 1:25 p.m. that it issued a temporary nationwide grounding at the airline's request while it worked to resolve a "reservation computer issue."
At 1:50 p.m., Southwest tweeted that it is in the process of resuming normal operations after a "system issue" Tuesday afternoon created flight disruptions throughout its network.
Southwest said customer wait times are longer than normal due to many customers needed assistance, and encouraged visiting its website to explore self-service options for resolving flight issues.
"Flight delays are likely, check directly with your airline for more information," DIA said in an earlier tweet warning of continued delays.
Southwest Airlines said about 12 p.m. Tuesday on Twitter, "We are aware of system issues and are working quickly to resolve. We will share more info soon."
Multiple 9NEWS viewers and people on Twitter from across the country said their Southwest Airlines flights experienced delays on Monday evening – and photos showed that some planes were parked on the tarmac at Denver International Airport.
A DIA spokesperson said "we are aware that Southwest Airlines is currently experiencing computer issues that are causing flight delays," and recommended reaching out to the company itself.
FlightAware.com, which tracks airlines across the country, reported 61 delays at DIA as of around 8:45 p.m. Monday. Dozens of other delays were also reported at the airports in Los Angeles, Dallas, Atlanta, Chicago and San Francisco.
Southwest has not responded to a 9NEWS request for comment. On Twitter, the airline told one traveler it is experiencing a "system error" and apologized for delays.
A 9NEWS viewer said he saw 14 Southwest planes on the tarmac at DIA.
Another Twitter user also said he had been stuck on a plane at DIA for "over an hour."
Another viewer said he was told "no gate for at least an hour" due to the "computer issues."
9NEWS Director of Content Tim Ryan tweeted a photo showing multiple passengers waiting at the Baltimore/Washington Airport for a flight to Denver to board.
Ryan later tweeted that his plane took off an hour and 25 minutes late.
It's unclear what caused the system outage.
This story will be updated once more information becomes available from Southwest.
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