Three masked men kill 12 people after storming the offices of satirical magazine Charlie Hebdo, then flee after a gun battle.
The bodies of victims are being removed from a Paris building as police conduct a major manhunt for gunmen who killed 12 at the headquarters of satirical magazine Charlie Hebdo.
Three masked gunmen stormed the offices of the controversial publication, which has previously been attacked for its portrayal of the Prophet Mohammed.
The attackers were armed with Kalashnikov rifles and a rocket-propelled grenade during the attack on Wednesday morning.
Police sources have said two brothers from the Paris region and another man from Rheims have been linked to the attack.
French President Francois Hollande has declared a national day of mourning tomorrow.
The attackers are said to have called out their victims by name before shooting them. In one video clip, one of them is heard to shout: "We have avenged the prophet."
The editor and a cartoonist for the newspaper, who went by the pen names Charb and Cabu, were among those killed.
Radio France chief executive Mathieu Gilet announced on Twitter that a contributor, Bernard Maris, was another of the victims.
Two police officers were also among the dead, including one assigned as Charb's bodyguard after death threats were made against him.
Another 11 people have also been injured, at least four seriously.
After the attack, the gunmen returned to their black Citroen getaway car and shouted: "We have avenged the Prophet Mohammed, we have killed Charlie Hebdo."
The vehicle was later abandoned and is being examined by forensics teams.
A police official said the gunmen headed towards the north-eastern Paris suburbs, before hijacking a white Renault Clio and driving in the direction of the Porte de Pantin - one of the main routes out of the French capital.
He said: "Those men are our most wanted, so that stays a priority, they're armed and extremely dangerous - and things will certainly evolve behind the scenes."
Shocking footage has emerged of a wounded police officer being shot in the head in cold blood as he lies on the ground, holding his hands up.
Another video taken from the top of a building near the magazine's offices shows a firefight in the street below.
In a televised address, he said: "We have to respond according to the crime, first of all by finding the authors of this infamy and we have to ensure that they are arrested, judged and... punished very severely.
"Everything will be done in order to apprehend them.
"We must also protect all public buildings... security forces will be deployed everywhere where there could be a threat.
"Our best weapon is our unity, the unity of all our citizens, nothing can divide us, nothing must separate us. Freedom will always be stronger than barbarism."
Hassen Chalghoumi, imam of Drancy mosque in the Paris suburb of Seine-Saint-Denis, said: "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."
An extra 3,000 police officers have been deployed on the streets in a massive security operation, according to reports, as the security threat level in France was raised to the highest level.
Parisiens turned out at 7pm on the Place de la Republique in a show of support for the victims and of the right of free speech.
Social media users have used the hashtag #jesuischarlie to show solidarity for the victims of the shooting, with the Charlie Hebdo website also using the image as its masthead.
Charlie Hebdo's editor-in-chief Gerard Biard told France Inter: "I don't understand how people can attack a newspaper with heavy weapons. A newspaper is not a weapon of war."
US President Barack Obama and the Russian leader Vladimir Putin both condemned the shootings.
David Cameron tweeted: "The murders in Paris are sickening. We stand with the French people in the fight against terror and defending the freedom of the press."
The British PM will have a "full intelligence briefing" on the attack from security agencies, said Downing Street, and the Home Secretary will chair a meeting of the emergency response committee COBRA on Thursday.
The Queen expressed her condolences for the victims of the shooting. In a tweet she said: "Prince Philip and I send our condolences to the families of those who have been killed and to those who have been injured in the attack in Paris this morning."
Spain has raised its security threat level in response to the attack.
Tens of thousands of people have attended public shows of solidarity in France and around the world in the wake of the attack.
The shooting is one of the worst terror attacks in France in decades.
In 1995, a bomb on a commuter train blamed on Algerian extremists killed eight people at the Saint Michel metro station in Paris.
Mohamed Merah, an al Qaeda-inspired gunman, killed seven people in Toulouse in 2012, including three French soldiers and three children.