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 magazineâ€™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 â€œof exceptional barbarityâ€.
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, where they hijacked a second car.
Witnesses said they heard the gunmen shouting â€œWe have avenged the Prophet Muhammadâ€ and â€œGod is Greatâ€ in Arabic (â€œAllahu Akbarâ€).
The number of attackers was initially reported to be two, but French Interior Minister Bernard Cazeneuve later said security services were hunting three â€œcriminalsâ€. He said that Paris had been placed on the highest alert.
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, as well as Charlie Hebdo contributor and French economist Bernard Maris.
The attack took place during the magazineâ€™s daily editorial meeting.
At least four people were critically wounded in the attack.
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 Hebdoâ€™s account was a cartoon of the Islamic State militant group leader, Abu Bakr al-Baghdadi.
Charlie Hebdoâ€™s website, which went offline during the attack, is showing the single image of â€œJe suis Charlieâ€ (â€œI am Charlie) on a black banner, referring to a hashtag that is trending on Twitter in solidarity with the victims.
People had been â€œmurdered in a cowardly mannerâ€, President Hollande told reporters at the scene. â€œWe are threatened because we are a country of liberty,â€ he added, appealing for national unity.
French government officials are holding an emergency meeting, and President Hollande is due to give a televised address later.
Footage shot by an eyewitness outside the magazineâ€™s office 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.
Eyewitnesses described seeing two black-hooded men entering the building carrying Kalashnikovs, with reports of up to 50 shots fired.
Gilles Boulanger, who works in the same building as the office, told French TV channel Itele: â€œThere 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. Youâ€™d think it was a war zone.â€
Wandrille Lanos, a TV reporter who works across the road, was one of the first people to enter the Charlie Hebdo office after the attack.
â€œAs we progressed into the office, we saw that the number of casualties was very high. There was a lot of people dead on the floor, and there was blood everywhere,â€ he told the BBC.
After the attack, which occurred at about 10:30 GMT, police warned French media outlets to be on alert and pay attention to security.
The country was already on the alert for Islamist militant 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 about 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.
BBC Online News