The websites of several Swedish government agencies were knocked offline on Friday.
The affected sites included those of the Riksbank, the Riksdag, the Swedish Institute, the Svea Court of Appeal, the Swedish police and security service Säpo.
Details of the attack were being discussed freely on Facebook by people claiming to be a part of Anonymous, reports said.
The Facebook group, called #OpPRK #OpPirateBay, said the attacks were in retaliation for police raids on a hosting company associated with both The Pirate Bay and Wikileaks.
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Terry Greer-King, UK managing director for internet security firm Check Point said the attacks demonstrate that a determined group can cause disruption to any organisation, even at government level, no matter how well-defended their networks and sites are.
"In May this year, we surveyed over 2,500 IT professionals worldwide and found that they rated DDoS (distributed denial-of-service) attacks as one of the top risks to their networks. Organisations need to be able to collaborate and share intelligence on emerging threats, so that the severity of attacks can be mitigated or even blocked before they can cause widespread damage,” he said.
In April, a study revealed that financial services firms were targeted by three times as many DDoS attacks in the first quarter of 2012 than in the previous three months.
This represented a 25% increase compared with the same period in 2011.
There was also a 3,000% increase in malicious packet traffic, according to the latest Quarterly Global DDoS Attack Report by security firm Prolexic Technologies.