The world’s best-known group of vigilante computer hackers has reportedly hit back at French extremists as revenge for the Charlie Hebdo killings.

A Twitter account claiming links to Anonymous, a loose affiliation of hackers, has claimed responsibility for denying access to a Muslim website, which has been criticised in French media for advocating terrorism.

“Jihad is of great importance” read one of the blocked website’s pages, which now redirects users to DuckDuckGo, the search engine reportedly preferred by Anonymous users.

• ‘Je suis Charlie’: World’s cartoonists take a stand
• Charlie Hebdo to publish one million copies
• Paris shooting: free speech in the firing line

The hacker group has reportedly declared war on French extremists, saying in an unverified statement: “We will fight always and everywhere the enemies of freedom of speech.”

Anonymous has no leaders and a fluid membership, which makes it difficult to verify a true Anonymous attack. Hacking targets are reportedly put to a vote amongst its membership and actioned if a majority agree.

The Twitter account @OpCharlieHebdo boasted about the attack on Sunday with the words “JeSuisCharlie” (“I am Charlie”), a statement of solidarity with the victims of the tragic shooting that has become one of the most popular hashtags in the history of social media.

#TangoDown : http://t.co/rHJrjTZ8mA Expect us. #JeSuisCharlie #OpCharlieHebdo #CharlieHebdo pic.twitter.com/RK7gBWr8QS

— OpCharlieHebdo (@OpCharlieHebdo) January 10, 2015

In a video statement posted on Twitter, a group claiming to be Anonymous mourns the four cartoonists “murdered for their opinions and the freedom of their newspaper” by “cowards”.

“[I]t is our responsibility to react,” says a person wearing a Guy Fawkes mask, the symbol of Anonymous, in a disguised voice.

“Anonymous reminds every citizen that freedom of the press is fundamental to all democracy.”

The hacked website Ansar Al-Haqq, which translates as ‘Defender of Truth’, has been called the face of Jihadism in France. Various YouTube accounts and blogs sharing the same name promote hard-line interpretations of Islam. One blog discusses fighting for Allah, how to avoid US drone attacks and imposing Sharia law on the world.