Anonymous created a Twitter account for the hack, calling it #OpParis, and created a new website for all ISIS accounts.
More than 5,500 Twitter accounts were taken down, reported Softpedia.
“We can not fight them with guns and rifles,” a spokesperson with Anonymous told the BBC. The spokesperson added, “Stopping their propaganda is an effective way to weaken their manpower and their presence in the Internet.”
The main announcement was made via a YouTube video, where a self-described Anonymous member wearing the group’s signature Guy Fawkes mask said in French, “Anonymous from all over the world will hunt you down. Expect massive cyber attacks. War is declared. Get prepared.”
— Anonymous (@GroupAnon) November 15, 2015
The masked person added that “we will launch the biggest operation ever against you.”
Another spokesman told RT.com that Anonymous has “people very, very close to ISIS on the ground,” without elaborating. The collective, he added, has tools that “might be better than any world government’s tools to combat ISIS online.”
Anonymous also claims to have “better hackers” than ISIS.
— Jack Phillips (@jackphillips5) November 18, 2015
“Cyberattacks can have a tremendous impact,” expert David Gewirtz told CBS News.
He added, “Of course, they can’t be used to arrest people or take terrorists off the field, but they can certainly be used to compromise structural components of terrorist operations. More to the point, they can go after both the money that terrorists have and their funding sources. Damaging the money flow can certainly have an impact on the terrorists’ operations.”
The terrorist attacks in Paris left more than 129 people dead and more than 300 people injured on Friday night.
Anonymous has spent more than a decade hacking corporate, religious, and government websites. After January’s terrorist attacks on the Charlie Hebdo publication, Anonymous started fighting ISIS-related extremists online. The collective got a list of more than 39,000 Twitter accounts.