The airline blamed the problems on a "temporary network outage" that began shortly after 12 p.m. PT today. United apologized for the disruptions and said it's allowing customers to "cancel or rebook their itineraries without penalty" if they were affected.
The Federal Aviation Administration had issued a "ground stop" for United flights flying into United's biggest hubs -- Houston, Newark, and San Francisco -- according to a USA Today report. FlightAware.com said that United suffered a "systemwide outage" of its passenger reservation and ticketing system, which ended at approximately 2:20 p.m. PT today.
As Twitter erupted with complaints from dismayed passengers facing long lines and missed connections a few hours ago, United responded by saying around 1 p.m. PT that it's "working to get systems back online."
A United spokesman contacted by CNET did not immediately respond with additional details.
The United.com Web site was up as of 2:30 p.m. PT. CBS 4 Denver reported a few minutes earlier that the site was displaying this error message: "We are experiencing technical difficulties. This will temporarily affect our ability to serve you online."
Moira Forbes, the publisher of ForbesWoman, posted a photo of a hand-written boarding pass that a United representative had given her.
Another Twitter user, Chris Elliott, who said he was stuck on the runway on a plane bound for Ireland, pledged that he will "will continue to rant about United Airlines till [sic] my plane takes off." About 30 minutes later, he posted: "Should be leaving soon. Thank God."
The FAA's Web site had said: "Due to UAL COMPUTER ISSUE, there is a Traffic Management (Ground Stop) Program in effect for traffic arriving SFO." By 3 p.m. PT, that message had disappeared.
Last updated at 3:30 p.m. PT