The outage affected at least Boston, the company said, but comments on Twitter pointed to service being down in several other cities including Washington, D.C., and Baltimore.
"Our engineers are working very aggressively to fix the problem," said Doreen Vigue, a Boston-based spokeswoman for Comcast. She said she didn't have any specific information about the cause of the fault or the cities affected beyond Boston.
Twitter postings point to a DNS (domain name system) server problem.
DNS is the system that converts human-friendly Internet address like "www.pcworld.com" into a numeric Internet address, which are vital to Internet communications. If the DNS servers go down there's no way for a user's PC to address data correctly and so communication stops.
Several users said their connections returned to normal when they switched their PC's settings from Comcast's DNS servers to those of Google or the Open DNS project.