Long lines formed at international airports across the United States, including Philadelphia’s, on Friday after U.S. Customs and Border Protection’s computer system broke down and dramatically slowed the processing of thousands of people arriving from abroad, including U.S. citizens.

Officials say they would not stop processing passengers but said they had to switch to a slower, alternative process.

It was not immediately clear how long people were having to wait, but reports on Twitter showed clogged terminals and extremely long lines. Among the airports affected was Philadelphia International Airport.

"U.S. Customs and Border Protection is experiencing a temporary outage with its processing systems at various air ports of entry and is taking immediate action to address the technology disruption," the Department of Homeland Security agency said in a statement. "CBP officers continue to process international travelers using alternative procedures until systems are back online. Travelers at some ports of entry are experiencing longer than usual wait times and CBP officers are working to process travelers as quickly as possible while maintaining the highest levels of security."

Reports of frustration emerged on Twitter on Friday afternoon, and Los Angeles International Airport said it dispatched its team "of Guest Experience Members to help at @CBP customs areas to help direct guests and provide other assistance."

“@CBP systems are experiencing an issue which appears to be impacting multiple airports including LAX,” the airport said. “Officers are processing passengers manually so please check with your airline for the latest status of any flight impacts. More details as they become available.”