The internet around the world has been slowed down in what security experts are describing as the biggest cyber-attack of its kind in history.
A row between a spam-fighting group and hosting firm has sparked retaliation attacks affecting the wider internet.
It is having an impact on popular services like Netflix - and experts worry it could escalate to affect banking and email systems.
Five national cyber-police-forces are investigating the attacks.
Spamhaus, a group based in both London and Geneva, is a non-profit organisation which aims to help email providers filter out spam and other unwanted content.
To do this, the group maintains a number of blocklists - a database of servers known to be being used for malicious purposes.
Recently, Spamhaus blocked servers maintained by Cyberbunker, a Dutch web host which states it will host anything with the exception of child ****ography or terrorism-related material.
Sven Olaf Kamphuis, who claims to be a spokesman for Cyberbunker, said, in a message, that Spamhaus was abusing its position, and should not be allowed to decide "what goes and does not go on the internet".
Spamhaus has alleged that Cyberbunker, in cooperation with "criminal gangs" from Eastern Europe and Russia, is behind the attack.