An Internet watchdog group responsible for keeping ads for counterfeit Viagra and bogus weight-loss pills out of inboxes around the world has been hit by a huge cyberattack, a crushing electronic onslaught that one expert said had already rippled across the Web.

Spam-fighting organization Spamhaus said Wednesday it had been buffeted by a massive "denial of service" attack since mid-March, apparently from groups angry at being blacklisted by the Geneva-based outfit.

"It is a small miracle that we're still online," Spamhaus researcher Vincent Hanna said in an interview. The interruptions came after Spamhaus temporarily added CyberBunker, a Dutch Web-hosting company, to a blacklist that is used by email providers to weed out spam.

Denial-of-service attacks overwhelm target servers with traffic. In a blog post, San Francisco-based CloudFlare Inc. said the attackers were taking advantage of weaknesses in the Internet's infrastructure to trick servers from across the Internet into routing billions of bits of junk traffic to Spamhaus every second.

The attack could be bad news for email users, many of whose incoming messages are checked against Spamhaus' widely used and constantly updated blacklists.

Hanna said that his site had so far managed to stay on top of the spammers, but he warned that being knocked offline could give them an opening to step up their mailings.

The New York Times, calling the disruptions "one of the largest computer attacks on the Internet," reported Wednesday that millions of Web users have experienced delays in services such as Netflix video streaming or couldn't reach a certain website for a short time.

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"The size of the attack hurt some very large networks and Internet exchange points such as the London Internet Exchange," John Reid, a spokesman for Spamhaus, said.