Customers were reporting issues with cable TV, Internet and phone service.
PHILADELPHIA (KYW Newsradio/CNN) -- Comcast customers across the United States were slowly coming back online late Friday after a massive outage left them without Internet, cable TV, and telephone service.
The outage started Friday morning and quickly spread to several areas, including Philadelphia, southern New Jersey, New York, Washington D.C. and Boston, even to cities as far away as San Francisco and Seattle.
Complaints peaked early in the day, but Comcast service problems continued popping up well into the afternoon. In some cases the outage lasted most of the afternoon, but Comcast said in a tweet Friday evening that it had started restoring service.
We identified 2 unrelated fiber cuts to our network backbone providers. Our engineers are restoring services to business & residential internet, tv & voice customers. Apologies to those affected. If still having issue, use https://t.co/P6Ac7dZ5h4 to DM.
The Philadelphia communications giant was trying to determine just what went wrong, but said in a statement that "one of Comcast's large backbone network partners had a fiber cut that we believe is also impacting other providers," including Verizon and AT&T.
Fiber optic cables are used for long distance high-speed data transmission.
The company could not say how many people were affected by outage, but many who were seemed unhappy and took to Twitter to complain about the issues. Some reported difficulty getting through to Comcast customer service. The company took to Twitter to provide updates and urged customers not to call 911.
CNN's Heather Kelly contributed to this report.