A Dutchman arrested in Spain last month in connection with an unprecedented cyberattack that reportedly slowed down the Internet, has been extradited to the Netherlands where he appeared before a judge on Wednesday, Dutch prosecutors said.

Sven Olaf Kamphuis, 35, was transferred to the Netherlands on Monday night following his arrest last month in Barcelona by the Spanish police.

"He appeared today before a Rotterdam judge," the Public Prosecutor's office said in a statement with Dutch national news agency ANP reporting he was remanded behind bars for another two weeks.

Prosecutors only identified the suspect by his initials "S.K." but a source close to the investigation said it was Kamphuis, who acted as a spokesman for the Dutch web-hosting service Cyberbunker at the time of the attack.

The late March attack targeted Spamhaus, a Geneva-based volunteer group that publishes spam blacklists used by networks to filter out unwanted email, and lead to cyberspace congestion.

Spamhaus blamed Cyberbunker for the attack.

Kamphuis describes himself on his Facebook page as Minister of Telecommunications and Foreign Affairs for the Cyberbunker Republic.

Computers, storage devices and mobile phones were seized during the Barcelona raid and prosecutors said Wednesday the investigation was continuing.

"The Dutch investigation into the DDoS attacks on Spamhaus is still going ahead full-steam, being led by police's Team High Tech Crime unit," the statement said.

So-called distributed denial of service attacks (DDoS) essentially bombard sites with traffic from various sources in order to disrupt or seize servers.

The attack was described as the most powerful ever seen and slowed web traffic.

The attacks began, according to Spamhaus, after it placed Cyberbunker, a web hosting firm that "offers anonymous hosting of anything except child porn and anything related to terrorism", on its blacklist.

Cyberbunker said it had been unfairly labelled as a haven for cybercrime and spam.

Spamhaus, which also has offices in London, essentially patrols the Internet to root out spammers and provides updated lists of likely perpetrators to network operators around the world.

Experts said the attacks flooded Spamhaus servers with 300 billion bits per second (300 gigabytes) of data.

Prior DDoS attacks have been measured at 50 gigabytes per second.

Because of the way Internet traffic flows, these DDoS attacks create congestion and ripple effects around the web.