"We are not scared anymore," read an online message that purported to be a response by Anonymous and splinter group Lulz Security.
"Your threats to arrest us are meaningless to us as you cannot arrest an idea... there is nothing -- absolutely nothing -- you can possibly do to make us stop."
As if to underscore the point, a message posted at a Twitter account by "AnonymousIRC" claimed to have looted about a gigabyte of NATO data that even the rebel hacker group deemed too sensitive to release.
"Yes, we haz (sic) more of your delicious data," the Twitter post read. "You call it war; we laugh at your battleships."
US authorities arrested 16 people for cyber crimes on Tuesday, including 14 over an online attack on the PayPal website claimed by Anonymous. British police arrested one suspect and Dutch police four.
The US indictment against the 14 hackers alleges the denial of service (DDoS) attacks on PayPal were "retribution" because the site terminated a donation account for the whistle-blowing group WikiLeaks.
Anonymous hackers called the PayPal attacks "Operation Avenge Assange," in reference to WikiLeaks founder Julian Assange, the indictment said, adding that the US raids were coordinated with police in Britain and the Netherlands.