Hackers released a video statement condemning the gun attack on the French magazine Charlie Hebdo.
The group Anonymous describes the short film as a "message for al-Qaeda, the Islamic State and other terrorists".
In it, a masked and hooded figure with a distorted voice threatens in French: "We are declaring war against you, the terrorists."
The video was uploaded to the group's Belgian YouTube account.
Seated in front of a desk holding a piece of paper, the figure claims the group will track down and close all accounts related to terrorists on social networks to avenge the French victims.
The video featured the hashtag #OpCharlieHebdo, meaning Operation Charlie Hebdo.
But critics say taking down extremist websites could make it harder for security forces to monitor terrorists.
Anonymous is a group made up of activists and hackers claiming to defend and protect democracy.
They've previously made threats against the BBC, while hackers linked to the group also threatened to release sex tape images which they claimed were of Iggy Azalea.
The group has carried out denial of service attacks (DDoS) on websites belonging to governments, as well as religious and corporate organisations.
Twelve people, including eight journalists and the magazine's editor, were murdered in last Wednesday's gun rampage.
On Friday, the two brothers suspected of being behind the shootings were killed in an assault at a warehouse where they had held a hostage north of Paris.
The hostage was freed unhurt.
At the same time, anti-terror forces stormed a Jewish supermarket in eastern Paris where several hostages were held by another gunman.
Four hostages at the supermarket and one captor were killed.
It's thought the three hostage-takers were connected and that the supermarket captor was also behind the shooting of a police woman on Thursday.
Police are still looking for a 26-year-old woman, Hayat Boumeddiene, over that attack.