Spam-fighting organization Spamhaus has experienced a large-scale DDoS attack over the past weekend and extending into this week. Security experts claim that it was one of the largest cyber attacks ever seen. Although the organization's web site and its mail were knocked down for a while, the data systems continued to work normally throughout the attack.

While the origin of the attacks has not been identified, some experts pointed the finger at Cyberbunker, possibly in coordination with Eastern European cyber-criminals.

Security firm CloudFlare, said the attackers changed tactics after the first layer of protection was implemented last week.

"Rather than attacking our customers directly, they started going after the network providers CloudFlare uses for bandwidth," Prince said.

"Once the attackers realized they couldn't knock CloudFlare itself offline... they went after our direct peers."

Prince said the so-called denial of service attack, which essentially bombards sites with traffic in an effort to disrupt, was "one of the largest ever reported."

"If the Internet felt a bit more sluggish for you over the last few days in Europe, this may be part of the reason why," he said in a blog post.

Prince noted that these attacks "are like bazookas and the events of the last week have shown the damage they can cause." . "What's troubling is that, compared with what is possible, this attack may prove to be relatively modest."

"Based on the reported scale of the attack, which was evaluated at 300 Gigabits per second, we can confirm that this is one of the largest DDoS operations to date," online security firm Kaspersky Lab said in a statement.

Spamhaus carries a constantly-updated blacklist of service providers suspected of offering refuge for spammers. CloudFlare estimates that Spamhaus "is directly or indirectly responsible for filtering as much as 80 percent of daily spam messages."