Gunmen have killed at least twelve people, after attacking the offices of a magazine that has repeatedly published articles and graphics which mocked Islam. A 2011 edition was told to have been guest edited by the Islamic prophet Muhammad and another issue included a poignant picture of the magazine’s editor kissing a Muslim man, with the message: “Love is stronger than hate.”

Masked gunmen stormed the magazine’s offices earlier this afternoon with assault rifles and opened fire while staff where in an editorial meeting.

Four of the magazine’s well-known cartoonists, including its editor, were among those killed, as well as two police officers. A major police operation is now under way to find the gunmen who fled by car. French Interior Minister Bernard Cazeneuve has said security services were hunting three “criminals”and that Paris had been placed on the highest alert.

President Francois Hollande said there was no doubt it had been a terrorist attack “of exceptional barbarity”.

British prime minister David Cameron called the attack an Breaking News“appalling terrorist outrage” and said he felt “huge sympathy” for the families of those killed, adding: “We must never allow the values we hold dear, of democracy, of freedom of speech, to be damaged by these terrorists. We must stand against what they have done.”

Ban Ki-moon, the UN secretary-general, has called the Charlie Hebdo attack a “direct assault on democracy, media and freedom of expression”. He went on: “We stand with the government and people of France … This horrific attack is meant to divide. We must not fall into that trap. This is a moment for solidarity … We must stand against the forces of division and hate.”

Witnesses said they heard the gunmen shouting “We have avenged the Prophet Muhammad” and “God is Great” in Arabic (“Allahu Akbar”).

The satirical weekly has courted controversy in the past with its irreverent take on news and current affairs. It was firebombed in November 2011 a day after it carried a caricature of the Prophet Muhammad.

The latest tweet on Charlie Hebdo’s account was a cartoon of the Islamic State militant group leader, Abu Bakr al-Baghdadi.