Gunmen have attacked the Paris office of French satirical magazine Charlie Hebdo, killing 12 people and injuring seven in an apparent Islamist attack.At least two masked attackers opened fire with assault rifles in the office and exchanged shots with police in the street outside before escaping by car.
The gunmen shouted "we have avenged the Prophet Muhammad", witnesses say.
President Francois Hollande said there was no doubt it had been a terrorist attack "of exceptional barbarity".
A major police operation is under way in the Paris area to catch the killers.
The latest tweet on Charlie Hebdo's account was a cartoon of the Islamic State militant group leader, Abu Bakr al-Baghdadi.
UK Prime Minister David Cameron said in a tweet: "The murders in Paris are sickening. We stand with the French people in the fight against terror and defending the freedom of the press."
Analysis: Hugh Schofield, BBC News, Paris
Charlie Hebdo is part of a venerable tradition in French journalism going back to the scandal sheets that denounced Marie-Antoinette in the run-up to the French Revolution.
The tradition combines left-wing radicalism with a provocative scurrility that often borders on the obscene. Its decision to mock the Prophet Muhammad in 2011 was entirely consistent with its historic raison d'etre.
The paper has never sold in enormous numbers - and for 10 years from 1981, it ceased publication for lack of resources.
But with its garish front-page cartoons and incendiary headlines, it is an unmissable staple of newspaper kiosks and railway station booksellers.