7 January 2015 Last updated at 15:06 GMT
Gunmen have shot dead 12 people at the Paris office of French satirical magazine Charlie Hebdo in an apparent militant Islamist attack.
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 under way to find three gunmen who fled by car.
President Francois Hollande said there was no doubt it had been a terrorist attack “of exceptional barbarity”.
The masked attackers opened fire with assault rifles in the office and exchanged shots with police in the street outside before escaping by car. They later abandoned the car in Rue de Meaux, northern Paris.
Witnesses said they heard the gunmen shouting “we have avenged the Prophet Muhammad”.
Charlie Hebdo editor Stephane Charbonnier, 47, had received death threats in the past and was living under police protection.
French media have named the three other cartoonists killed in the attack as Cabu, Tignous and Wolinski. The attack took place during the magazine’s daily editorial meeting.
At least seven people were wounded in the attack, with several in critical condition.
The satirical weekly has courted controversy in the past with its irreverent take on news and current affairs. It was fire-bombed 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.
People had been “murdered in a cowardly manner”, President Hollande told reporters at the scene. “We are threatened because we are a country of liberty,” he added, appealing for national unity.
US President Barack Obama has condemned the “horrific shooting”, offering to provide any assistance needed “to help bring these terrorists to justice”.
UK Prime Minister David Cameron said in a tweet: “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 Arab League and Al-Azhar mosque, Egypt’s top Islamic institution, have also condemned the attack.
Analysis: Hugh Schofield, BBC News, Paris
Charlie Hebdo is part of a venerable tradition in French journalism going back to the scandal sheets that denounced Marie-Antoinette in the run-up to the French Revolution.
The tradition combines left-wing radicalism with a provocative scurrility that often borders on the obscene. Its decision to mock the Prophet Muhammad in 2011 was entirely consistent with its historic raison d’etre.
The paper has never sold in enormous numbers – and for 10 years from 1981, it ceased publication for lack of resources.
But with its garish front-page cartoons and incendiary headlines, it is an unmissable staple of newspaper kiosks and railway station booksellers.
A bullet impact at the Charlie Hebdo office in Paris
Police and rescue services near the office of Charlie Hebdo
Footage shot by an eyewitness shows two armed men dressed in black approach a wounded police officer lying on a pavement. One of the men shoots the officer in the head, before both men are seen running back towards a black vehicle and driving away.
An eyewitness, Benoit Bringer, told French TV channel Itele: “Two black-hooded men entered the building with Kalashnikovs. A few minutes later we heard lots of shots.”
The men were then seen fleeing the building.
Gilles Boulanger, who works in the same building as the office, told the same channel: “A neighbour called to warn me that there were armed men in the building and that we had to shut all the doors.
“And several minutes later, there were several shots heard in the building from automatic weapons firing in all directions. So then we looked out of the window and saw the shooting was on Boulevard Richard-Lenoir, with the police. It was really upsetting. You’d think it was a war zone.”
After the late morning attack, police warned French media outlets to be on alert and pay attention to security.
The country was already on the alert for Islamist attacks after several incidents just before Christmas.
Cars were driven at shoppers in two cities, Dijon and Nantes, and police were attacked by a man wielding a knife in Tours.
While the French government denied the attacks were linked, it announced plans to further raise security in public spaces, including the deployment of around 300 soldiers.
Danish newspaper Jyllands-Posten, which published cartoons of the Prophet Muhammad in 2005 sparking riots in Muslim countries, says it has stepped up security in the wake of the Charlie Hebdo attack.
The number of attackers was initially reported to be two, but French Interior Minister Bernard Cazeneuve later said they were hunting three “criminals”.