Sean Duffy, the secretary of transportation, confirmed the outage in a social media post, citing “a communication issue impacting all Air Route Traffic Control Centers.”

“FAA technicians are investigating the issue,” he added. “We will provide immediate updates as we have them.”

The agency posted an advisory stating that “all airports are ground stopped,” citing “extraordinary circumstances and for reasons of safety” as the reasons.

“All departing traffic should remain ground stopped until further notice,” the official advisory read, adding that the landings of inbound international flights would be managed on a case-by-case basis.

Shortly after the advisory was posted, an F.A.A. website that lists air traffic control system notices appeared to have gone down, erasing dozens of notices that previously had been listed.

The stoppage, which began shortly before 4 p.m., could affect hundreds of flights already airborne, in addition to airplanes on the ground waiting to take off. As of about 4:45 p.m., there were more than 7,800 flights delayed in the United States and about 350 cancellations, according to FlightAware, a flight tracking website.

More than 2 million people pass through airport security checkpoints on a typical day, according to Transportation Security Administration data. On Thursday, the agency screened nearly 2.5 million people. Amid delays and cancellations on Friday evening, many passengers were taken off flights or told to go home.

According to FlightRadar24, an aviation tracking site, there were more than 1,000 airborne aircraft bound for U.S. airports during the nationwide ground stop. According to the flight tracking app Flighty, there were more than 30,000 departures and arrivals scheduled for the remainder of the day.

At least some of the airborne flights appeared to be landing amid the stoppage. About half an hour after the ground stop was declared, planes could be seen landing at O’Hare International Airport.

But passengers waiting to take off were stuck in limbo.

In the Denver airport, passengers were taken off their flights with their suitcases. A handful began rebooking travel for the next day. But most waited patiently for any kind of update. A Southwest flight attendant announced that the airline expected an update in 30 minutes.

Before the advisory was published, the F.A.A. halted flights to Chicago’s O’Hare and Midway airports, citing equipment problems, and reported equipment-related departure delays at Newark, Las Vegas, Teterboro, Orlando, Houston and Phoenix.

Friday’s ground stop is only the third such nationwide moratorium on flights in recent history. The F.A.A. issued nationwide ground stops previously on Sept. 11, 2001, in the wake of the terrorist attack on the World Trade Center in New York, and on Jan. 11, 2023, due to an outage in the F.A.A.’s system to alert pilots to safety issues.

Here’s what else to know:

• If your flight is canceled: You are entitled to a refund if your flight is canceled in the United States, regardless of the reason, or if a flight is significantly delayed or changed and you decide not to take it. Here are other travel tips that might be helpful. Read more ›

• A busy travel month: The ground stop is happening about two weeks before Thanksgiving, one of the busiest times of the year for air travel. On average, U.S. airlines are expected to fly about 2.7 million passengers each day over the holiday period, according to industry trade group Airlines for America.

• The last ground stop: The last nationwide ground stop came in the early morning of Jan. 11, 2023, and lasted around 90 minutes. The disruption came after a system to alert pilots of real-time dangers went down. Read more ›

Christine Chung, Niraj Chokshi, Luke Broadwater, Lisa Lerer and Robert Chiarito contributed reporting.