One expert warned that the electronic onslaught was affecting others across the Internet.
Users could experience slower Internet or be subjected to unwanted emails.
Spamhaus carries a constantly-updated blacklist of service providers suspected of offering refuge for spammers.
In an interview, Spamhaus' Vincent Hanna said his site had been hit by a crushing wave of denial-of-service attacks and that it was "a small miracle that we're still online."
Hanna said his group had been weathering such attacks since mid-March. The attacks work by flooding target servers with traffic.
Patrick Gilmore of Akamai Technologies said the attack was so large that online bystanders had been hit as well.