Twelve people are reported to have been killed after gunmen opened fire at the publication’s Paris office. Agence France Presse reported that among the victims were the editor, Stephane Charbonnier, and three cartoonists - Jean Cabut, Bernard Verlhac and Georges Wolinski.
There were also media reports that the gunmen shouted "Allahu Akbar" at the scene of the massacre.
The left-leaning Charlie Hebdo has caused controversy for mocking Islam and in 2007 it reprinted cartoons of the Prophet Mohammad, first published in a Danish newspaper.
World leaders condemned the attack, with David Cameron calling it "sickening" while German Chancellor Angela Merkel labelled the incident "abominable".
Speaking at the scene, French President Francois Hollande called the incident a "terrorist attack of the most extreme barbarity".
White House spokesman Josh Earnest told CNN: "It's not just an attack on the people of France, it's an attack on some of the basic values we hold dear in this country - freedom of speech, freedom of expression and a free press."
In a statement issued this afternoon, NUJ general secretary Michelle Stanistreet said: "The assassination of journalists at Charlie Hebdo, cynically targeted on press day to maximise casualties, is an attempt to assassinate the free press.
"Our hearts go out to the families of the 10 journalists and police officers killed in this despicable raid. The newspaper had already been the subject of attacks by people who want to suppress democracy and freedom of speech. These journalists have now paid with their lives; the perpetrators must swiftly be brought to justice.
"Supporters of free speech and civil liberties must stand together with governments to condemn this act and defend the right of all journalists to do their job without fear of threats, intimidation and brutal murder."