The reactions came after Paris Prosecutor Francois Molins confirmed the deaths and said 10 others had also been injured after unidentified gunmen opened fire at the offices as staff attended an editorial meeting.
President Francois Hollande said at the crime scene the raid was "an attack on liberty".
He said: "This is an act of exceptional barbarism ... against journalists ... who have always endeavored to show that, in France, you can operate to defend your ideas."
In Washington, President Barack Obama offered condolences to the victims and said he had directed his administration "to provide any assistance needed to help bring these terrorists to justice".
He said in a statement: "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."
Russian President Vladimir Putin expressed sympathy and support for the families and friends of the victims, condemned "the cynical crime" and reiterated his "readiness to continue active cooperation in combating the threat of terrorism".
The President of the European Commission, Jean-Claude Juncker, expressed his deep sympathy and "solidarity" with France, saying it was "an intolerable act, a barbarism that concerns us all as human beings and Europeans".
Speaking at the House of Commons in London, David Cameron said that the United Kingdom "opposed to all forms of terrorism".
He said: "The murders in Paris are sickening. We stand with the French people in the fight against terror and defending the freedom of the press."
German Chancellor Angela Merkel has condemned the attack in a telegram of condolences to French President Hollande.
"In these difficult hours we are standing side by side with our French friend," she said.
"This heinous attack has targeted freedom of expression and press freedom, which are core elements of a free and democratic culture. This is an attack, for which there is no justification,” Merkel added. 'Evil attack'
NATO chief Jens Stoltenberg said in a statement: "All NATO allies stand together in the fight against terrorism. This was a barbaric act and an outrageous attack on press freedom."
Ed Miliband, the leader of the UK's opposition Labour Party, described the event as an "evil attack by terrorists intent on attacking our democratic way of life and freedom of speech.
"We are united in our determination to defeat them."
Indian Prime Minister Narendra Modi also condemned the attack and Greek Prime Minister Antonis Samaras said: "On behalf of Greek citizens, I express my deep grief and outrage for the deadly terrorist attack in Paris.
"We Greeks are in solidarity with the families of the innocent victims and participate in the mourning of our friends, the French people.”