The site, www.polisen.se crashed after receiving around half a million page requests a second. Police IT director Lars Lindahl told The Local that the problem was caused by a distributed denial of service attack. He said that investigators had not made a positive link to The Pirate Bay case:
"We don't know who is behind it or where they are based, but an investigation is underway."
He said the attack was caused by a virus spread to computers around the world, which simultaneously tried to reach the Swedish police website.
"The page requests are coming from completely innocent people all over the world."
Police were emphasising that their systems had not been hacked, and sensitive information had not been compromised. Staff still have access to the force's internal network and the Internet.
"This is manageable as long as there is not a major incident during which we need to use the website to provide information to the public," said Lindahl.
The attack slowed down briefly on Friday morning, allowing visitors to reach the website for a short period. From 11.30 am the site was once more unreachable.
By 6pm on Friday the site was operational again.