Cyber-activists attacked the websites of Finnish anti-piracy groups after a local Internet service provider was forced to block access to a popular file-sharing website, officials said Tuesday.

Antti Kotilainen, a spokesman for the Copyright Information and Anti-Piracy Centre (CIAPC), told AFP that websites run by his organisation and the International Federation of the Phonographic Industry (IFPI) had been "down since Monday".

"It isn't the first time this has happened ... We have no precise information who is behind this," he added.

Meanwhile a group of hacker activists, or "hacktivists," known as Anonymous Finland claimed responsibility on the microblogging site Twitter for the attacks, insisting about the CIAPC site: "We'll keep it down as long as we want".

In May 2011, the CIAPC and IFPI brought a case against Elisa, a major Finnish Internet service provider, seeking to block its customers from accessing the Swedish file sharing website The Pirate Bay.

Following a ruling by the Helsinki District Court last October ordering Elisa to block access to Pirate Bay or face a 100,000-euro ($130,000) fine, the service provider announced Monday it would temporarily block access to the site. The websites of both plaintiffs in the anti-piracy case later became inaccessible, apparently due to the "Elisagate" campaign launched by the cyber-activists claiming an anti-censorhip agenda.

"We know about it ... We are following what's happening," police investigator Timo Piiroinen told AFP, adding that such cases are "extremely challenging."

Elisa meanwhile indicated in a customer statement Monday that it intended to appeal the October court ruling.

Founded in 2003, The Pirate Bay makes it possible to skirt copyright fees and share music, film and computer game files using bit torrent technology, or peer-to-peer links offered on the site.

The site has been involved in numerous lawsuits concerning copyright infringement, and Finland is just one of several countries that have attempted to block access to the site.