Several Swedish government websites could not be accessed Friday after they had received a warning the evening before from a group claiming to be the Anonymous collective, which supports fugitive WikiLeaks founder Julian Assange.
The Swedish branch of Anonymous denied it was behind the attacks.
Assange is wanted by the Swedish authorities for questioning over allegations of sexual assault but has been under diplomatic protection inside the Ecuadoran embassy in London since mid-August.
At midday the web sites of the Swedish Central Bank, the intelligence services, the Parliament and the courts could not be reached, AFP determined.
They may have been the victims of a distributed denial of service (DDoS) attack in which sites are saturated by requests.
The Swedish branch of Anonymous, which communicates on the micro-blogging Twitter site under the name of @AnonOpsSweden, denied it was involved.
"We call b(expletive) on the op... #nothingNew," the message said in English.
"Our theory on #opsweden its European Cyber Security Month/EU testdriving cyberattacks blaming it on #Anonymous," it added in a later message.
In a film posted on the online version of the tabloid newspaper Aftonbladet Friday a young man aged 18 said it was he who had published a video Thursday evening on the Internet site YouTube.
In it a masked individual read in English in a female voice a message indicating that the sites of several Swedish public bodies would be attacked Friday.
"It was I who made the film," the young man said in the Aftonbladet contribution. "But I had help with the wording. I'm not so good in English," he added.
"We are going to attack the Central Bank and the police...and several other government sites," the young man said. "It'll be the worst thing ever done by Anonymous. It'll mark world history."
All week Swedish public authorities and companies have experienced similar attacks. Police have linked them to the Assange affair.
Internet attacks blocked access to several popular Swedish websites for part of Monday.
Explore further: Hackers apparently hit Swedish government site