Many young Swedes support Pirate Bay's file-sharing system
Sweden's Pirate Party says it has had a surge in membership, giving its leaders hope that anti-corporate feeling will translate into electoral success.
The party is campaigning in the June European elections, fielding 20 candidates. It wants sweeping reform of copyright law and an end to patents.
The membership boost followed the jailing last week of four founders of a file-sharing website, The Pirate Bay.
The party says membership soared from 14,711 to 36,624 in just a few days.
Party leader Rick Falkvinge told BBC News that "in terms of membership it is now the fourth largest party in Sweden".
"If everybody who is angry about the Pirate Bay verdict votes then we'll get at least one MEP," he said, referring to the elections for the European Parliament.
Hundreds of supporters of Pirate Bay demonstrated in the Swedish capital Stockholm on Saturday, some of them waving Jolly Roger flags.
In the verdict against the world's most high-profile file-sharing website, the Stockholm district court found Frederik Neij, Gottfrid Svartholm Warg, Carl Lundstrom and Peter Sunde guilty of breaking copyright law and sentenced them to a year in jail.
They were also ordered to pay $4.5m (£3m) in damages.
Record companies welcomed the verdict, but the men plan to appeal.
Mr Falkvinge said support for the Pirate Party was not only coming from young people, but also "a considerably older crowd saying 'enough is enough'."
"They see how these laws break civil liberties in an unacceptable way," he said.
He said the party membership was split about 50-50 between people aged under 25 and over.
But its youth section is "by far the largest" among Swedish political parties, he added.
The party says it has only three issues on its agenda: "to fundamentally reform copyright law, get rid of the patent system and ensure that citizens' rights to privacy are respected".
Mr Falkvinge said it was funded entirely by private donations, typically of 100 kronor (£8; $12).
The Pirate Bay, with an estimated 22 million users, is reckoned to be the most popular of the many file-sharing sites on the internet.
Its success angered executives and artists in the music, TV and film industries, who saw the Swedish case as an important step in stamping out the illegal sharing of copyrighted media.