"After complaints from the public we have been able to establish that there is child pornography on Pirate Bay," wrote the head of the National Criminal Investigation Department's IT crimes unit Stefan Kronkvist in a statement.
He said that The Pirate Bay would be placed on the National Police Board's blacklist if the material was still in place next week, but no formal decision has yet been taken. The blacklist means that Sweden's internet operators, of which there are 15 including Telia, Tele 2 and Bredbandsbolaget, could block access to the site.
Instead of arriving at the site, visitors would be redirected to a page where they would be informed that the site was used for distributing child pornography.
Fredrik Neij, one of the site's founders, called the police's threat "abuse" and "a mistake". He said that there are no indications from users that child pornography is accessible via the site.
"During my time here there have been 7-10 cases, and in those we have gone in as soon as possible and removed the files. And we usually get a great many responses from our users if there is any child pornography," he said.
According to Neij, the police have not contacted The Pirate Bay.
"The first thing they should have done was to tell us that there was child pornography, so that we could check it out and if necessary remove the files. The second thing was that they should try to get hold of whoever is spreading the files."
Neij said that there are around 600,000 'torrent files' on The Pirate Bay which can be shared between users.
"It's not possible for us to check if any of them contain child porn. But as soon as it happens there are many users who will alert us right away."
The Pirate Party protested strongly against the police's measures. Party leader Rick Falkvinge said it was a "scandalous abuse" of power and not worthy of a law-abiding country.
"Child porn filters should not be used by authorities to shut down undesirable sites which are not breaking any laws," said Falkvinge.