cyber-attack in history.
A row between an anti-spam group and a web-hosting company sparked retaliation attacks affecting the wider internet.
It is had an impact on popular services like Netflix - and boffins worry it could escalate to affect banking and email systems.
Five national cyber-police-forces are investigating the attacks.
Spamhaus, a non-profit group based in both London and Geneva which helps email providers filter out spam, added servers maintained by Cyberbunker, a Dutch web host, to their blocked list.
Spamhaus alleged that Cyberbunker, in cooperation with “criminal gangs” from Eastern Europe and Russia, then launched a Distributed Denial of Service attack.
This is where the intended target is flooded with large amounts of traffic in an attempt to render it unreachable.
The cyber-attack is thought to have hit the Spamhaus servers with up to 300 billion bits per second (300Gbps) of data, making it the largest cyber attack in history.
Prof Alan Woodward, a cybersecurity expert at the University of Surrey, said the effect of the attack was slowing internet services across the world.
H e said: “If you imagine it as a motorway, attacks try and put enough traffic on there to clog up the on and off ramps.
“With this attack, there’s so much traffic it’s clogging up the motorway itself.”
Steve Linford, chief executive for Spamhaus, said: “We’ve been under this cyber-attack for well over a week.
“But we’re up - they haven’t been able to knock us down.”
Mr Linford told the BBC that the attack was being investigated by five different national cyber-police forces around the world.
He said the attack’s power would be strong enough to take down government internet infrastructure.
“If you aimed this at Downing Street they would be down instantly,” he said. “They would be completely off the internet.”
