One of most high-profile trials over copyright infringement in years began today in Sweden. Four men behind The Pirate Bay website – which enables people to find others willing to share audio, video, games and other files with them – appeared in court in Stockholm to answer the charge that they had assisted in copyright infringement.

The film, music and games industries are saying that the defendants not only encouraged copyright infringement but also profited from it, while the defendants argue that they hosted no shared files and therefore are not responsible for infringement.

The Pirate Bay is a "torrent" tracker, which uses the peer-to-peer file sharing technology called BitTorrent. Trackers don't host the music, video or software files themselves, but allow users to search for and download "torrent" files. Those in turn allow users to find other people who have the file they want, and to share the files amongst themselves. Each BitTorrent user with a copy of the file contributes a piece to others who are downloading it.

BitTorrent filesharing has been enormously popular: some internet service providers reckon that around 80% of the data traffic passing over their networks uses the service.

This morning, prosecutor Håkan Roswall outlined his case, accusing the four men of profiting from promoting copyright infringement via one of the largest filesharing services on the internet. An estimated 22 million people have used The Pirate Bay site.

Roswall said in court that the defendants were aware that Swedish law was changed in 2005, incorporating an EU directive that makes it illegal to download copyright protected files from the internet.

The defendants include three of the website's co-founders, Fredik Neij, Gottfrid Svartholm Warg and Peter Sunde Kolmisoppi. Prosecutors also charged one of the site's donors, Swedish dotcom millionaire Carl Lundström.

Lundström has donated money to the organisation, and the prosecution says that in 2004 he helped the other defendents configure larger numbers of computers to host the site.

If convicted, the men face two years in prison and a fine of 1.2m kronor (£98,000).

Lundström's lawyer said: "To supply a service that can be used illegally or legally is not illegal."

The first day of the trial was such an event that Swedish newspaper Dagens Nyheter's media blog reported that tickets to get into the courtroom were selling on the black market for 500 Swedish kronor (£41).

However, the intense interest and festival atmosphere outside of the court was not matched by high courtroom drama. Most of the day was taken up by the prosecutor showing various screenshots of The Pirate Bay site demonstrating how to download from the site. As Snild Dolkow who is translating Swedish coverage of the trial into English put it:

On the first day of trial, the prosecutor gave to me..

one screenshot of a torrent download,

another screenshot of a torrent download,

another screenshot of a torrent download,

another screenshot of a torrent download,

another screenshot of a torrent download,

another screenshot of a torrent download,

another screenshot of a torrent download,



.... and so on.

While BitTorrent is used to share copyrighted files, the technology is also commonly used to distribute software such as the open-source operating system Linux because it lessens the bandwidth costs for distributors of the software: everyone with a copy of the file contributes a piece to the overall download.

Plaintiffs in the case include Warner Bros, MGM, Colombia Pictures Industries, 20th Century Fox, Sony BMG, Universal and EMI.

Led by the International Federation of the Phonographic Industry, the plaintiffs are calling for damages of 120 million kronor (£9.8m). The chairman of the IFPI, John Kennedy, said: "The evidence in this case will show that The Pirate Bay is a commercial business which made substantial amounts of money for its operators, despite their claim to be only interested in spreading culture for free."

The prosecution will rely on expert witnesses, emails between the defendents and invoices sent to advertisers.

Pirate Bay held a press conference on Sunday, bringing in supporters on buses painted with pirate ship. The supporters carried banners with "ctrl+c,ctrl+v", the keyboard shortcuts for copying and pasting text, images or files.

At the press conference, defendant Gottfrid Svartholm Warg said the music and movie industry "are welcome to send me a bill". If he does get one, he said he will frame it and hang it on his wall.

Follow the coverage online via the Technology blog.