Eleven people have been shot dead in an attack at the offices of a French satirical weekly which angered some Muslims after publishing crude caricatures of Islam's Prophet Mohammed.
Police said several masked gunmen stormed the headquarters of Charlie Hebdo in Paris shooting indiscriminately.
Prime Minister David Cameron joined the condemnation of the attack, saying: "The murders in Paris are sickening. We stand with the French people in the fight against terror and defending the freedom of the press."
Deputy Prime Minister Nick Clegg said the killings were a "barbaric attack on freedom of speech".
"My thoughts are with the victims, their families and their colleagues," he said.
The publication has launched a series of attacks on Muslim extremism and the last tweet on its profile page @Charlie_Hebdo_, sent about an hour before the shootings, included a satirical cartoon of Islamic State leader Abu Bakr al-Baghdadi.
In it he wishes everyone "good health".
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