Cable provider's Web service went down in Boston, Washington, D.C., and other major cities along the East Coast.
Server problems knocked out Internet service to millions of Comcast customers along the Eastern Seaboard late Sunday.
The glitch hit users in Massachusetts, Connecticut, Maryland, Virginia, New York, New Hampshire, and D.C., according to media reports. Service had apparently been restored as of early Monday.
Comcast said the problem was confined to the company's high-speed Internet service, and did not impact its digital TV and telephone offerings. "All other services are working properly," a Comcast spokesman told NBC News.
"We certainly apologize for any inconvenience this may be causing our customers," the spokesman said.
Word of the trouble first spread across social networking sites like Facebook and Twitter. Comcast itself turned to Twitter to update customers on the status of the outage. Customer service rep Bill Gerth, who tweets under the name ComcastWill, reported that repairs were underway late Sunday.
"It appears our engineers are finalizing the correction. I appreciate everyone's patience during this unexpected outage," Gerth wrote in a Twitter post at about 9 p.m. Sunday.
Earlier, Gerth made a post that ominously hinted at the scale of the disruption. "Internet outage larger than just Boston, we should have a resolution shortly," Gerth tweeted around 8 p.m.
The outage could hardly have come at a worse time for many customers, as no doubt many were planning to peruse the Web late Sunday in preparation for Cyber Monday sales, many of which began at Midnight.
Some Comcast customers reported they were able to work around the problem by switching to an alternate DNS, such as OpenDNS, in order to circumvent Comcast's apparent DNS issue.