Rickard Falkvinge, leader of The Pirate Party - which is trying to reform laws around copyright and patents in the digital age - told the BBC that the verdict was "a gross injustice".
"This wasn't a criminal trial, it was a political trial. It is just gross beyond description that you can jail four people for providing infrastructure.
"There is a lot of anger in Sweden right now. File-sharing is an institution here and while I can't encourage people to break copyright law, I'm not following it and I don't agree with it."
"Today's events make file-sharing a hot political issue and we're going to take this to the European Parliament."
The Pirate Bay is the world's most high profile file-sharing website and was set up in 2003 by anti-copyright organisation Piratbyran, but for the last five years it has been run by individuals.
Millions of files are exchanged using the service every day.
No copyright content is hosted on The Pirate Bay's web servers; instead the site hosts "torrent" links to TV, film and music files held on its users' computers.
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