"Anonymous from all over the world will hunt you down. You should know that we will find you, and we will not let you go. We will launch the biggest operation ever against you," the spokesman says in the video.
The masked speaker also hints at "massive cyberattacks", adding: "War is declared. Get prepared. The French people are stronger than you and will come out of this atrocity even stronger."
The hacktivist group is already running the #OpISIS campaign and has exposed hundreds of accounts of alleged IS supporters on Twitter. It also sends geo-location of these accounts to security agencies in Europe and the US. Earlier this year, it had also revealed the identities of many US and UK companies hosting pro-IS websites on their servers.
Anonymous hackers had also threatened Isis after a deadly attack on the satirical magazine Charlie Hebdo in Paris in January 2015.