The interior minister, Bernard Cazeneuve, said three men were being hunted and said “all the means” had been mobilised to “neutralise the three criminals who have committed this barbaric act”.
He added that the operation will take place as quickly as possible in order to “identify the aggressors and arrest them in a way that they will be punished with the severity that corresponds to the barbaric act they have committed”.
The masked attackers, armed with automatic rifles, were heard shouting “Allahu Akbar” - God is great - as they stormed the office before opening fire in an editorial meeting.
A lorry tows the car used by armed gunmen who stormed the Paris offices of satirical newspaper Charlie Hebdo
Footage showed them shouting in French “we have killed Charlie Hebdo -we have avenged the Prophet Mohamed,” in an apparent reference to the magazine’s publication of controversial cartoons depicting the Muslim prophet.
One journalist who was in the magazine’s office during the shooting texted a friend to say he was alive, according to The Daily Mail. “There is death all around me. The jihadists spared me,” he said.
Benoit Bringer, a journalist who works in adjoining office, said: “There were very many people in the building. We evacuated via the roof just next to the office.”
Witnesses said the gunmen claimed to be part of terrorist group al-Qaeda in Yemen and asked for cartoonists by name before murdering them.
The gunmen fled eastwards towards the Paris suburbs, dumping their car in a residential area, police said. They then hijacked another car before running over a pedestrian and disappearing.
“There is a possibility of other attacks and other sites are being secured,” police union official Rocco Contento said.
Eight journalists, a guest and two police officers were killed, said Paris prosecutor Francois Molin. Eleven more were injured, including four who are in a critical condition.
Gérald Kierzec, 40, a doctor who was one of the first people to go into the magazine’s office after the shooting, told The Daily Telegraph: “There was a first body lying in the lobby. Then I took the stairs which were covered in blood. When I got to the second floor, there were bodies lying one on top of another.
“It was carnage, with war wounds. There was blood everywhere. I have never seen anything like it in my career.”
Charb, the editor of Charlie Hebdo (Reuters)
Footage taken by terrified witnesses from windows and on rooftops overlooking the scene showed the terrorists shooting one of their victims, a police uniform at point-blank range as he lay injured on the pavement.
The Times reported that the injured police officer, who was lying on the ground, said to one of the gunmen: “Do you want to kill me?” The man replied “OK, chief” and then shot him dead.
Corinne “Coco” Rey, a Charlie Hebdo cartoonist, told French newspaper L'Humanite: “I had gone to collect my daughter from day care and as I arrived in front of the door of the paper's building two hooded and armed men threatened us. They wanted to go inside, to go upstairs. I entered the code.
"They fired on Wolinski, Cabu...it lasted five minutes... sheltered under a desk...They spoke perfect French...claimed to be from al-Qaeda."
Prominent cartoonists Jean Cabut, the magazine’s artistic director, Stephane Charbonnier, its editor, and Bernard "Tignous" Verlhac were among the dead.
The massacre was France's deadliest terror attack in at least two decades and prompted condemnation from world leaders including David Cameron and Barack Obama, alongside journalists and free speech campaigners.
The Prime Minister, who described the killings as "sickening", met Angela Merkel over the tragedy and said the leaders had contacted President Hollande to offer their support.
Ms Merkel, who is in London on an official visit, condemned the shooting as "barbarous", while President Obama condemned the "horrific" attack.
It has prompted a wave of global solidarity with Charlie Hebdo over what is being seen as a direct attack on freedom of expression.
The hashtag #jesuisCharlie is trending on Twitter and people are wearing stickers bearing the slogan at vigils in Paris and around the world.
Additional reporting by agencies