World leaders have condemned a shooting at the office of a French newspaper which killed 11 people, labelling it an “abominable” attack on expression and free press.

Three armed gunmen stormed the Paris offices of satirical newspaper Charlie Hedbo on Wednesday morning local time, killing both journalists and police.

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Thousands of police officers were scouring the capital city for the gunmen who were still on the run after reportedly escaping in a hijacked car.

French President Francois Hollande said there was “no doubt” the shooting was a terrorist attack, and declared a national day of mourning on Thursday.

Emergency workers at the scene. Photo: AAP

He confirmed 11 people had been killed in the attack, with another four in critical condition.

“An act of exceptional barbarism has been committed in Paris against a newspaper. A paper, in other words, an organ of free speech,” Mr Hollande said.

“Today France is in shock; the shock of a multiple assassination, a terrorist attack.”

Mr Hollande revealed several terrorist plots had been foiled in the last few weeks, saying “these are tense times”.

In a tweet, the French leader said freedom would always be stronger than barbarism.

In a statement, Prime Minister Tony Abbott said Australia would stand with the people of France during this “difficult” time.

“Freedom of expression is the cornerstone of a free society,” Mr Abbott said.

“The thoughts of all Australians are with the families of those who have lost their lives in this barbaric attack.”

An unspeakable atrocity in Paris overnight. If you don’t like something, you don’t read it; you don’t kill people you disagree with.

— Tony Abbott (@TonyAbbottMHR) January 7, 2015

British Prime Minister David Cameron said the attacks were sickening “murders”.

“The murders in Paris are sickening. We stand with the French people in the fight against terror and defending the freedom of the press,” Mr Cameron tweeted.

Speaking in parliament, he added: “This country stands united with the French people in opposition to all forms of terrorism and we stand squarly for free speech and democracy and these people will never be able to take us off these values.”

The murders in Paris are sickening. We stand with the French people in the fight against terror and defending the freedom of the press. — David Cameron (@David_Cameron) January 7, 2015

US President Barack Obama said he would offer any assistance needed to “bring these terrorists to justice”.

“France is America’s oldest ally, and has stood shoulder to shoulder with the United States in the fight against terrorists who threaten our shared security and the world,” Mr Obama said.

“For us to see the kind of cowardly evil attacks that took place today reinforces once again why it’s so important for us to stand in solidarity with them just as they stand in solidarity with us.”

A spokesperson for the Vatican said the shooting was an “abominable” act of violence against people and freedom of speech, a sentiment echoed by German leader Angela Merkel, who is in London for talks with Mr Cameron.

Angela Merkel and I met today. We’re united in our condemnation of the horrifying Paris murders. pic.twitter.com/oSM2us6I8I

— David Cameron (@David_Cameron) January 7, 2015

“It is also an attack on freedom of speech and the press, core elements of our free democratic culture. In no way can this be justified,” Ms Merkel said.

UN secretary-general Ban Ki-moon said the attack was meant to divide and asked people “not fall into that trap”.

“This is a moment for solidarity around the world. We must stand strong for freedom of expression and tolerance and stand against the forces of division and hate,” Mr Ban said.