A supporter of Anonymous, the loose confederation of rogue hackers, claimed responsibility on Monday for an attack that brought down GoDaddy, a Web hosting service, and the sites of its customers.

GoDaddy’s site was out for several hours Monday afternoon, as were many of the Web sites belonging to its 10.5 million customers, like the fantasy sports site Draftday and eBaum’s World, a humor site.

“We’re aware of the issues affecting our site,” Elizabeth Driscoll, a GoDaddy spokeswoman, said. “We’re still working to figure out what happened and determine the number of Web sites impacted.”

The person behind the Twitter account @AnonymousOwn3r took responsibility for the attack and said all of GoDaddy’s servers were knocked out with what is known as a distributed denial of service attack, or DDoS attack, in which a site is flooded with traffic until it collapses under the load.

@film_girl I’m taking godaddy down bacause well i’d like to test how the cyber security is safe and for more reasons that i can not talk now — Anonymous Own3r (@AnonymousOwn3r) 10 Sep 12

“When I do some DDoS attack, I like to let it down by many days,” the person claiming responsibility said on Twitter. “It can last one hour or one month.”

This person said that he had conducted the attack alone and that it was not part of a broader Anonymous campaign. GoDaddy briefly became a target of Anonymous last December for its initial support of the Stop Online Piracy Act, or SOPA, a Congressional bill that would have made it possible for copyright owners to seek court orders to take sites offline for practicing or aiding piracy. GoDaddy withdrew its support of the bill after customers began transferring their Web sites to its competitors and hackers threatened to attack it.

But one of the Twitter accounts most frequently associated with Anonymous, @AnonyOps, distanced the group from Monday’s attack and said it was the work of one individual. “He’s either a newbie to activism and cutting his teeth by doing this, which is misguided, or he’s trying to give Anons a bad reputation,” a Twitter message to a reporter from @AnonyOps said.

Godaddy technician trips over ethernet cord, pulling it from edge router. Blames Anonymous. — Anonymous (@AnonyOps) 10 Sep 12

Rich Miller, the editor in chief of Data Center Knowledge, an online publication that tracks the data center industry, said a shutdown at GoDaddy had widespread repercussions for the Internet.

“This is potentially the largest kind of problem you could have on the Internet,” Mr. Miller said. “GoDaddy is the largest company in the domain industry thanks to its colorful Super Bowl commercials.”

Mr. Miller estimated that GoDaddy managed 52 million domain names and hosted about 5 million Web sites on its servers. Its sheer size has made the company a ripe target for hackers, he said.

GoDaddy was previously the target of two notable DDoS attacks, one in 2007 and another in 2009.

GoDaddy customers vented their frustration on Twitter and elsewhere. Bob de Luna, a director of public information at the United Hospital Fund, a nonprofit health organization, said four domain names it had registered with GoDaddy had been down for several hours. He said that when he contacted GoDaddy’s customer service line, a representative told him 750 people were waiting in line behind him. The representative told him that during service failures, a typical wait list was 20 people.

GoDaddy’s Web site was still down Monday evening, and the company said it would provide updates through its Twitter and Facebook accounts.