Anonymous has warned about repercussions following arrests of three individuals suspected of being key members of the hacking group.
Arrests of the unnamed men were made in three Spanish cities - Barcelona, Valencia and Almeria - earlier this week.
AnonOps, the hacking group, referencing events in Spain, simply wrote "Expect us" on both a blog post and a Twitter status update.
Spain's Technological Investigation Brigade (BIT) alleged the men had operated a cell of Anonymous, helping run attacks against notable firms like Sony as well as on websites run by Governments of various nations, including Egypt and Iran.
BIT also posted images on Twitter of IRC conversations appearing to show plans to attack Spanish police websites and the electoral board with DDoS attacks.
Arrests of people suspected of Anonymous involvement have been made across the world, including in the UK.
How long is it going to take before they retaliate? It's not if, but when.
On previous occasions when Anonymous has been targeted, it has responded by fighting fire with fire.
When security firm HBGary said it had acquired details on senior members of the hacking collective, Anonymous acted ferociously by hacking the company and dumping tens of thousands of corporate emails on the web.
Luis Corrons, technical director of PandaLabs, said it was just a matter of time before Anonymous would respond.
"How long is it going to take before they retaliate? It's not if, but when," Corrons told IT PRO.
"They usually react in the same way, so DDoS against who? Well, the website of the Spanish police probably."
The arrests came thanks to legal changes in Spain made in December last year, which made DDoS attacks illegal in the country, Corrons said.
"There is a lack of law in some fields, especially in cyber security," Corrons added.
As for how to deal with hacking groups like Anonymous and suspected spin off LulzSec, there is no consensus in the industry.
"The security industry is really worried," Corrons added.
