The European Parliament's website fell under a distributed denial-of-service attack (DDOS) on Thursday in what the organisation classified as retaliation for the shutdown of the Megaupload file-sharing site and an anti-counterfeiting trade agreement.
The Parliament issued a statement saying it had acted to reduce the impact of the attacks, but the site was still down as of mid-afternoon Thursday, but has since come back online.
Anonymous, a loose-knit group of hackers and digital activists, has undertaken a series of DDOS attacks against government websites and other organisations following last week's international take-down of Megaupload, whose operators are wanted by US authorities for alleged copyright infringement related offenses.
Anonymous, which has sought to corral support from Internet users, created Web-based tools that allow non-technical people to participate in DDOS attacks, which bombard websites with an excessive amount of traffic, causing them to be unreachable.
Targets in the last week have included Universal Music, the US Department of Justice and the Recording Industry Association of America.
The hacktivist group has also been stirred to action by the Anti-Counterfeiting Trade Agreement (ACTA), a treaty that establishes a framework for how countries should deal with what are considered significant infringement of intellectual property rights.
The treaty is in the process of being ratified by countries. Several European Union countries - including Poland, which saw widespread protests over the treaty - signed the treaty on Thursday in Japan.
Eight other countries - Australia, Canada, Japan, South Korea, Morocco, New Zealand, Singapore, and the United States -signed ACTA last October.