The European sites of online shopping portal Amazon suffered a temporary failure on Sunday and media reports said the attacks were in retaliation to withdrawal of services to whistleblower site WikiLeaks.
A group of cyber activists called Anonymous started the Operation Payback campaign this week to conduct a denial-of-service (DDoS) attack on Amazon for denying services to the whistleblower site.
Soon after, Amazon's British, French, German, Austrian and Italian sites went down for about 30 minutes.
DDoS attacks, which are illegal in the UK, involve overloading a website with high numbers of requests so it stops working.
The websites of Paypal, Mastercard and Visa as well as the Swedish prosecutor's website (which passed an arrest warrant against WikiLeaks founder Julian Assange on charges of sexual wrongdoing) were also attacked.
However, on Friday, the group publicly announced that it has abandoned plans to hit Amazon, citing insufficient "forces".
There were no clear indications that Anonymous was responsible for the disruption to Amazon, and the group has not said it carried out an attack.
WikiLeaks, meanwhile, refused to associate itself with Anonymous, saying "we neither condemn nor applaud these attacks".