Anonymous, a loose-knit international network of activist hackers, is preparing to unleash waves of cyber attacks on Islamic State following the attacks in Paris last week that killed 129 people, a self-described member said in a video.
A man wearing a Guy Fawkes mask appeared on a video posted to YouTube and said the Islamic State militants who claimed responsibility for the Paris attacks were "vermin" and Anonymous would hunt them down.
"These attacks cannot remain unpunished," the man said, speaking in French.
"We are going to launch the biggest operation ever against you. Expect many cyberattacks. War has been declared. Get ready," the man said, without giving details of what the attacks would involve. "We don't forgive and we don't forget."
The video posted to YouTube had attracted more than 1.1 million views by 09:30 et on Monday.
Anonymous is an international network of activist computer hackers which has claimed responsibility for many cyberattacks against government, corporate and religious websites over the past dozen years.
Since the attack on French weekly Charlie Hebdo last January, which led to the deaths of 17 victims, Anonymous activists have waged an online vigilante campaign to force the shutdown of Twitter profiles suspected of belonging to ISIS supporters.