The target was Spamhaus, a Geneva-based spam fighting volunteer group, with the attack leading to heavy cyberspace congestion which may have affected the overall internet, according to US security company CloudFlare's Matthew Prince.
According to Spamhaus the attacks began last week after the group added Dutch-based hosting site Cyberbunker to its blacklist, leading Cyberbunker to argue it was being unfairly labelled as a haven for cybercrime and spam.
While the origin of the attacks has not been identified, some experts are pointing the finger at Cyberbunker, possibly in co-ordination with Eastern European cyber-criminals.
"Rather than attacking our customers directly, they started going after the network providers CloudFlare uses for bandwidth," Prince said.
"Once the attackers realised they couldn't knock CloudFlare itself offline ... they went after our direct peers."
Five different cyber-police-forces from around the world are responding to the attack, with Russian spam servers being investigated as well as Cyberbunker.
The New York Times quoted Sven Olaf Kamphuis, who claimed to be a spokesman for the attackers, as saying that Cyberbunker was retaliating against Spamhaus for "abusing their influence."
Prince described the denial of service attack as "one of the largest ever reported".
"We've seen congestion across several major Tier 1 (networks), primarily in Europe where most of the attacks were concentrated, that would have affected hundreds of millions of people even as they surfed sites unrelated to Spamhaus or CloudFlare," he said.
"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.
"They are like bazookas and the events of the last week have shown the damage they can cause," he said.
"What's troubling is that, compared with what is possible, this attack may prove to be relatively modest.
It is unclear as to whether the attacks have ceased or are still ongoing, more updates to come.