Anonymous – an international network of activists and hacktivists – has posted a video on YouTube saying it is waging cyberwar on jihadists.
A masked individual vows to avenge the victims of the attack, not through violence but by using a distributed denial of service attack to block access to jihadist websites.
The video, posted in French on the group's Belgique channel, follows the attack on the office of Charlie Hebdo magazine in Paris, in which 12 perople were killed.
In a statement it posted on pastebin, the group said freedom of expression had suffered an "inhuman assault”, and that it was their “duty to react”.
"It is clear that some people do not want, in a free world, this inviolable and sacred right to freely express opinions. Anonymous will never let this right be violated by obscurantism and mysticism. We will always fight the enemies of freedom of expression everywhere,” it said.
"Freedom of expression and opinion is a non-negotiable thing to tackle - to attack it is to attack democracy.
"Expect a massive and head-on reaction on our part because the defence of those freedoms is the foundation of our movement."