AT&T said on Thursday that three-quarters of its disrupted network had been restored, after a widespread outage temporarily eliminated service for users across the United States for hours.

“Some of our customers are experiencing wireless service interruptions this morning,” AT&T said in a statement. “We are working as quickly as possible to restore service to remaining customers.”

It was unclear how many customers were still affected or when service would be fully restored.

The outage, which affected people in cities including Atlanta, Los Angeles and New York, was first reported around 3:30 a.m. Eastern time, and cellular service and internet problems were still being reported eight hours later, according to Downdetector.com, which tracks user reports of telecommunication and internet disruptions.

AT&T said in an earlier statement that users should rely on Wi-Fi calling until service was restored.

A spokesman did not respond to questions about what had caused the outage or when service would be back. By 11 a.m., AT&T’s website showed that outages were limited to users in California, though users in other states were still reporting issues.

Reports surfaced early Thursday that FirstNet, the network AT&T maintains for emergency services personnel, had experienced outages, but AT&T said around 10:30 a.m. that the network was fully operational.

By around 12:30 p.m., more than 20,000 people were reporting to Downdetector that they were experiencing problems with AT&T, down from a peak of more than 70,000 at 9 a.m. Some users said that their phones showed “SOS” in the top corner, which signals the device is able to only make emergency calls. Verizon experienced 3,000 reports and T-Mobile about half that. Earlier, AT&T’s website had showed outages across the country, including in San Diego, Miami and Richmond, Va., with the initial cause listed as “maintenance activity.”

Verizon and T-Mobile said in statements that their networks were operating normally.

“Some customers experienced issues this morning when calling or texting with customers served by another carrier,” Verizon said. “We are continuing to monitor the situation.”

In an email, T-Mobile said that it did not experience an outage. “Downdetector is likely reflecting challenges our customers were having attempting to connect to users on other networks.”

Cricket, which is owned by AT&T, also reported that its users were experiencing wireless service interruptions and said it was working to restore service.

A spokesman for the Federal Communications Commission said its Public Safety and Homeland Security Bureau was “actively investigating” the outage and was in touch with AT&T as well as other providers.

John Kirby, a National Security Council spokesman, said on a call with reporters on Thursday that the Biden administration was told “that AT&T has no reason to think this was a cybersecurity incident,” although he added that they would not be certain until an investigation had been completed.

Mr. Kirby said that, in addition to the F.C.C., the Department of Homeland Security and the F.B.I. were collaborating with technology companies to investigate the outage.

The San Francisco Fire Department said on social media that it was aware of an issue affecting AT&T users who were trying to call 911. “We are actively engaged and monitoring this,” the fire department said. “If you are an AT&T customer and cannot get through to 911, then please try calling from a landline.”

Cities urged residents to find alternate ways of reaching emergency or municipal services, like landlines or phones connected to Wi-Fi. The City of Upper Arlington, Ohio, said the Fire Department might not be notified of fire alarms because of the outage. It urged that any fire alarm be followed up with a 911 call.

The Massachusetts State Police said on social media on Thursday morning that 911 call centers across the state had been flooded with calls from people checking to see if the emergency service worked from their phones. “Please do not do this,” the police said. “If you can successfully place a non-emergency call to another number via your cell service then your 911 service will also work.”

Debra Maddow, who lives in southwest Houston, said that she first noticed something was off after 7 a.m., when she went to check traffic and Google Maps was offline. Later, she visited a Starbucks to make an urgent call through its free Wi-Fi, she said.

“I’m really frustrated that they’re not telling us anything,” Ms. Maddow said in a phone interview over Wi-Fi. She said she tried to call AT&T for an update, but after a long time on hold, the call was dropped.

Victor Mather contributed reporting.