Governments lined up to offer their support after masked men armed with Kalashnikov automatic rifles opened fire at the offices of French satirical magazine Charlie Hebdo, in what President Francois Hollande said was a "terrorist attack".
The Committee to Protect Journalists said it was a "brazen assault on free expression in the heart of Europe", while Reporters Without Borders called it a "black day".
He offered US government help in tracking down the culprits, and added: "Time and again, the French people have stood up for the universal values that generations of our people have defended.
"France, and the great city of Paris where this outrageous attack took place, offer the world a timeless example that will endure well beyond the hateful vision of these killers."
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British Prime Minister David Cameron condemned the shooting as "sickening" and "barbaric", while German Chancellor Angela Merkel called it "despicable"; sentiments reflected across European capitals.
Israeli Foreign Minister Avigdor Lieberman also sent his condolences, saying: "Israel identifies with France's pain".
"Terror and terrorists must never be permitted to sow fear in the free world and the West must stand united and determined in the face of this danger," he said.
There was also condemnation from Muslim states of the gunmen.
The massacre took place after years of confrontation between Charlie Hebdo and Islamists infuriated by what they saw as the publication's attacks on their religion.
Al-Azhar, Sunni Islam's most prestigious centre of learning, called the attack "criminal" and said "Islam denounces any violence", according to Egyptian news agency MENA, while the Arab League also condemned the attack.
"We must fight against increasing racism, xenophobia and Islamophobia in Europe which threaten all our values. We must also fight against any form of terrorism," he said.