French police said at least 12 people, including two police officers, are dead after an attack at the Paris headquarters of the satirical weekly paper Charlie Hebdo.

Late Wednesday morning, masked gunmen stormed the weekly's offices, bursting into an editorial staff meeting and opening fire with automatic rifles.

Ten members of the Charlie Hebdo staff died in the attack, prosecutors said. Among the dead were four cartoonists, including co-founder Jean “Cabu” Cabut and editor-in-chief Stephane “Charb” Charbonnier.

The gunmen said "Allahu akbar" and that they were avenging the Prophet Muhammed, Paris Prosecutor Francois Molins told reporters.

After dozens of shots rang out, two assailants were seen calmly leaving the scene. One police officer was seen being shot as he lay wounded.

A police union official said the assailants - Interior Minister Bernard Cazeneuve said there were three - remained at large and there were fears of further attacks.

The government declared the highest state of alert, increasing security at transport hubs, religious sites, media offices and department stores as the search for the assailants got under way.

Another 20 people were injured in the attack, including four or five critically. Police union official Rocco Contento described the scene inside the offices as “carnage.”

There was no immediate claim of responsibility for the attack.

Charlie Hebdo is well-known for courting controversy with satirical attacks on political and religious leaders and has published numerous cartoons ridiculing the Prophet Muhammad. Jihadists online have repeatedly warned that the magazine would pay for its ridicule.

President Francois Hollande, who will address the nation on television at 8:00 p.m. (1900 UTC), rushed to the scene of what appeared to be a carefully planned attack. Sirens could be heard across the city.

“An act of indescribable barbarity has just been committed today in Paris,” Hollande said. He then defended the media: "No act of barbarity will ever extinguish freedom of the press."

France last year reinforced its anti-terrorism laws and is already on alert after calls from Islamist militants to attack its citizens and interests in reprisal for French military strikes on Islamist strongholds in the Middle East and Africa.

“I am extremely angry. These are criminals, barbarians. They have sold their soul to hell. This is not freedom. This is not Islam and I hope the French will come out united at the end of this,” said Hassen Chalghoumi, imam of the Drancy mosque in Paris's Seine-Saint-Denis northern suburb.

Chalghoumi, who also went to the site of the attack, said of the gunmen: “We must be firm with them, because they want terror, they want racism, they want to pit people against each other.”

"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," President Barack Obama said Wednesday, adding that America's thoughts and prayers are with the victims of the terrorist attack.

The White House said Obama has directed the administration to provide any assistance needed "to help bring these terrorists to justice."

In a joint appearance with Polish Foreign Minister Grzegorz Schetyna, Secretary of State John Kerry condemned Wednesday's Paris attack, saying, "Each and every American stands with you today."

Kerry said the U.S. stands with France in "solidarity and commitment" in "confronting extremism."

He also addressed the people of France in French.

Schetyna said his country is "deeply touched" by the attack, saying, "We stand together with France today."

German Chancellor Angela Merkel also condemned the shooting.

“This abominable act is not only an attack on the lives of French citizens and their security. It is also an attack on freedom of speech and the press, core elements of our free democratic culture," Merkel said.

The New York-based Committee to Protect Journalists described the attack as a brazen assault on free expression.