WASHINGTON — Parts of the Internet erupted Sunday in a battle among hackers, as factions disrupted a loosely co-ordinated "contest" among other groups trying to vandalize thousands of Web sites around the world.

Unknown attackers for hours knocked offline an independent security Web site, zone-h.org, that was verifying reports of online vandalism and being used by hackers to tally points for the competition. U.S. government and private technology experts warned last week that such vandalism was likely.

Three such vigilante-style attacks forced the hacker organizer, who identified himself only as "Eleonora 1/8673/8," to extend the contest until 6 p.m. EDT Sunday. But with continued attacks disrupting the ability of vandals to claim credit for their break-ins, some experts said it could be later this week before damage from this weekend's hacking would be known.

By late Sunday afternoon, hackers claimed responsibility for vandalizing hundreds of mostly obscure Web sites, such as ones for Security Title Co. and the Heart of Montana Realty Services, both of Bozeman, Mont.

"We're just a teeny little company," said Barb Asper, whose husband, Don, is one of the owners for Montana Realty. In a telephone interview, Don Asper called the attack "bothersome," before he and his wife contacted the firm's Web site provider to have the vandalized page replaced.

There were no reports of vandalism involving flagship Internet sites most consumers would recognize, a testament to improved online security over the past decade inside large corporations, government agencies and organizations.

"We at least knew it was coming," said Peter Allor of Internet Security Systems Inc., which distributed a warning Wednesday about the contest using an alert system for technology companies. "We took some efforts to harden our sites."

Some of the relatively unknown Web sites vandalized Sunday, such as one for a church in California, were converted into informal reporting mechanisms for identifying other hacked sites.

Many security experts said the nickname of the hacker claiming responsibility for the contest, "Eleonora 1/8673/8," was unfamiliar.