A battle among hackers erupted on the Internet yesterday as some factions disrupted a loosely coordinated effort among other groups trying to vandalize Web sites around the world

Unknown attackers for hours knocked offline an independent security Web site 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. After three such vigilante-style attacks, the hacker organizer extend the contest until 6 p.m. yesterday. 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 the weekend's hacking would be known. Hackers claimed responsibility yesterday for vandalizing hundreds of small Web sites, such as sites for Security Title Co. and the Heart of Montana Realty Services, both of Bozeman, Mont. Don Asper, an owner of Montana Realty, called the attack "bothersome," before contacting the firm's Web site provider to have the vandalized page replaced. There were no reports of vandalism involving larger, more well-known Internet sites, which may be a testament to improved online security at large companies, government agencies and organizations.