A hacker group known only as 'Anonymous' has vowed to exact vengeance against terrorist groups threatening the freedom of expression following the slaying of Charlie Hebdo journalists in Paris.

The 'hacktivists' are believed to be responsible for attacks against corporate, religious, and government agencies which they consider oppressors of democracy.

In their message, uploaded to the group's Belgian YouTube account, a member wearing their trademark Guy Fawkes mask, said that they are going to track down and shut down websites and social media accounts related to terrorist activities.

"We are declaring war against you, the terrorists," the masked figure said in French. "We will track you down - every last one - and will kill you. You allowed yourselves to kill innocent people, we will therefore avenge their deaths.

"You will not impose your sharia law in our democracies, we will not let your stupidity kill our liberties and our freedom of expression," the group declared. "We have warned you; expect your destruction."

The video also featured the words #OpCharlieHebdo, which stands for Operation Charlie Hebdo.

The video was released along with a statement on text sharing website Pastebin in which the group described the "cowardly and despicable act" in Paris.

"Freedom of expression and opinion is a non-negotiable thing to tackle - to attack it is to attack democracy," the statement added.

It further said that their group will engage in a massive retaliation as they consider themselves defenders of freedom, which they consider the foundation of their movement.

"We will track you everywhere on the planet, nowhere will you be safe. We are Anonymous. We are legion. We do not forget. We do not forgive. Be afraid of us, Islamic State and Al Qaeda - you will get our vengeance," they said.

The group joins many other groups and organisations who have condemned the killing of 12 journalists from Charlie Hebdo, a French satirical newspaper that features articles, cartoons, reports and jokes that are mostly leftist in nature.

The attack is believed to be in response to a series of cartoons mocking the Prophet Muhammad. Among those who perished was the magazine's former editor Stephane Charbonnieralso known as Charb.