The Chinese ring, which affected more than 2,000 advertisers, was discovered by US-based click fraud monitoring service Anchor Intelligence.
The large amount of members in the ring spread out the click fraud, thus making it harder to detect.
Members set up websites and signed up with ad networks, then clicked on the ads to trigger pay-per-click payments.
In the two weeks Anchor observed it the ring generated more than $3m of fraudulent clicks across 200,000 different IP addresses, according to details of Anchor's findings released to yesterday.
Richard Sim, vice-president of Anchor, said: "We have seen 200 fraud rings and this one by far trumps them all."
Separately, online advertising auditor Click Forensics said yesterday that a network of compromised PCs, known as a "botnet", that was known to be responsible for generating fake clicks is more sophisticated than previously thought.
After it takes over a computer the Bahama botnet redirects to Google.com traffic to a fake page hosted in Canada that copies the real Google page.
It is somehow able to respond to search queries but the results it delivers are not organic.
Instead when a user clicks on a listing he is routed through an ad network and it is treated as a cost per click ad, and the click will not always take the user where he wanted to go.
: "We have found that [the Bahama botnet] acts as a sort of perverted 'Robin Hood' among ad networks by robbing ad revenue from the top-tier players and delivering fraudulent traffic to second and third-tier ad networks and publishers."