“Anonymous everywhere in the world is going to hunt you down,” a hooded figure in black, wearing the group’s signature Guy Fawkes mask said in French.
It was not possible to verify its authenticity, but the statement and video had the hallmarks of the hacktivist network, known for mounting cyber attacks against government and corporate websites.
“Our country, France, was hit in Paris on November 13 around 2200 (2100 GMT) by multiple terrorist attacks claimed by you, the Islamic State,” the figure said in a gravelly, computer-altered voice.
“We are going to launch the biggest operation every mounted against you — get ready for a multitude of cyber attacks. War has been declared.”
The video, posted the day after the attacks, had garnered more than 1.3 million views by Monday afternoon.
The nearly three-minute clip opens with thundering orchestral music, displaying the Anonymous logo of a suited figure with a question mark for a head.
The speaker, seated like a news presenter, is flanked by the logo on one side and black-and-white news footage of the aftermath of the attacks, on the other.
France led a minute’s silence observed around the world on Monday in memory of the victims of the worst-ever terror attacks on French soil.
President Francois Hollande and his cabinet, all dressed in black, bowed their heads at the Sorbonne University in Paris, surrounded by scores of students.
The attacks — at six locations in and near Paris — came less than 11 months after jihadists struck satirical magazine Charlie Hebdo and a Jewish supermarket, killing 17.