At least 12 people have been killed in a shooting incident at the Paris office of French satirical weekly Charlie Hebdo, police have confirmed.

Witnesses said that at least two gunmen were involved, and that they were seen armed with AK-47s and pump-action shotguns. The attackers themselves are still on the run.

Four of France’s best-known satirical cartoonists, including the magazine's editor, were among the dead. A further 12 people were injured, some gravely, in what is the worst terrorist incident in France for 40 years.

In pictures: Charlie Hebdo shooting Show all 25 1 /25 In pictures: Charlie Hebdo shooting In pictures: Charlie Hebdo shooting Charlie Hebdo shooting Police investigators search for evidence as an unidentified man is detained (L) during an operation in the eastern French city of Reims, after the shooting against the Paris offices of Charlie Hebdo In pictures: Charlie Hebdo shooting Charlie Hebdo shooting A bullet impact is seen in a window of a building next to the French satirical newspaper Charlie Hebdo's office, in Paris AP In pictures: Charlie Hebdo shooting Charlie Hebdo shooting Police set up a piece of cloth at the back of a truck as they carry out a body from the offices of French satirical newspaper Charlie Hebdo in Paris Getty Images In pictures: Charlie Hebdo shooting Charlie Hebdo shooting Police block the roads next to the 'Charly Hebdo' headquarter where a shoutout occurred in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting A truck tows the car used by armed gunmen who stormed the Paris offices of satirical newspaper Charlie Hebdo Getty Images In pictures: Charlie Hebdo shooting Charlie Hebdo shooting An injured person is evacuated outside the French satirical newspaper Charlie Hebdo's office in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting French former Youth and Associations Junior minister Jeannette Bougrab (C) is comforted by an unidentified person outside of the headquarters of the French satirical newspaper Charlie Hebdo in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting French Justice Minister Christiane Taubira reacts outside of the headquarters of the French satirical newspaper Charlie Hebdo in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting French soldiers patrol in front of the Eiffel Tower in Paris as the capital was placed under the highest alert status after heavily armed gunmen stormed French satirical newspaper Charlie Hebdo and shot dead at least 12 people in the deadliest attack in France in four decades In pictures: Charlie Hebdo shooting Charlie Hebdo shooting Firefighters carry an injured man on a stretcher in front of the offices of the French satirical newspaper Charlie Hebdo in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting A victim is evacuated on a stretcher after armed gunmen stormed the offices of the French satirical newspaper Charlie Hebdo in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting A French Policeman is shot in the head in the street as two masked gunmen stormed the Charlie Hebdo headquarters, opening fire on staff In pictures: Charlie Hebdo shooting Charlie Hebdo shooting A bullet's impact on the window of the offices of the French satirical newspaper Charlie Hebdo in Paris, after armed gunmen stormed the offices leaving at least 10 people dead In pictures: Charlie Hebdo shooting Charlie Hebdo shooting A police car riddled with bullets during an attack on the offices of the newspaper Charlie Hebdo in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting French President Francois Hollande (C) arrives after a shooting at the Paris offices of Charlie Hebdo In pictures: Charlie Hebdo shooting Charlie Hebdo shooting France's interior minister Bernard Cazeneuve (C, L) and Paris' Mayor Anne Hidalgo (C, R) arrive at the headquarters of the French satirical newspaper Charlie Hebdo in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting Firefighters and police officers gather in front of the offices of the French satirical newspaper Charlie Hebdo in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting Firefighters and police officers gather in front of the offices of the French satirical newspaper Charlie Hebdo in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting Armed gunmen face police officers near the offices of the French satirical newspaper Charlie Hebdo in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting A police officer stands next to the bicycle of a police officer who was hit by a car near the shell of a bullet (bottom R) not far from the offices of the French satirical newspaper Charlie Hebdo in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting Firefighters carry an injured man on a stretcher in front of the offices of the French satirical newspaper Charlie Hebdo in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting Police forces gather in street outside the offices of the French satirical newspaper Charlie Hebdo in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting Firefighters carry an injured man on a stretcher in front of the offices of the French satirical newspaper Charlie Hebdo in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting Firefighters carry an injured man on a stretcher in front of the offices of the French satirical newspaper Charlie Hebdo in Paris In pictures: Charlie Hebdo shooting Charlie Hebdo shooting Police officers and firefighters gather in front of the offices of the French satirical newspaper Charlie Hebdo in Paris, after armed gunmen stormed the offices leaving "casualties", according to the publication's cartoonist, and "six seriously injured" police officers according to City Hall

Police said that at least 10 of the killings took place in the initial attack inside the building, or died within minutes because of the seriousness of their injuries.

As the gunmen left in a black Citroën hatchback, they met with a police car which was part of the permanent protection given to the magazine after it was firebombed in 2011. The attackers got out of their vehicle and opened fire, killing two officers.

Video footage posted to social media showed armed gunmen running through the streets of Paris, shooting with automatic weapons and shouting "Allahu Akbar". According to an eye-witness, one of the gunmen shouted: "The Prophet is avenged."

Below is an edited version of one of the videos.

Warning: Viewers may find the following video distressing

Paris has raised its terror alert to the highest setting in the aftermath of the attack, while a manhunt has begun for those involved. A social media post from France's AFP news agency reported that the men may also have been armed with at least one rocket-launcher, though this was not corroborated by other reports.

France's interior minister Bernard Cazeneuve said police were looking for "three criminals", though it was not clear how many took part in the shooting itself. All resources had been mobilised, he said, "to neutralise those who committed this barbaric act".

A Charlie Hebdo cartoonist who survived the attack described how two "hooded and armed men" forced her to enter the code letting them into the magazine's offices.

A victim is evacuated on a stretcher after armed gunmen stormed the offices of the French satirical newspaper Charlie Hebdo in Paris

Corinne "Coco" Rey told the French newspaper L'Humanite that the attack lasted five minutes. She said the assailants "spoke perfect French" and "claimed to be from al-Qaeda".

The French President Francois Hollande has travelled to the scene in Paris's 11th arrondissement after what he called a "terrorist attack of the most extreme barbarity".

He said it was the latest in a series of terrorist incidents on French soil, and that the nation was in a state of shock.

French President Francois Hollande (C) arrives after a shooting at the Paris offices of Charlie Hebdo

Firefighters and police officers gather in front of the offices of the French satirical newspaper Charlie Hebdo in Paris

"This is a difficult moment for France," Hollande told reporters. "We have prevented several attacks. We knew that we were still under threat because we are a country that cherishes freedom."

The urgent priority for officials on Wednesday afternoon remained tracking down those responsible. Paris police said the gunmen abandoned their car at the northern Porte de Patin and escaped.

Mr Cazeneuve said the attack had set in motion protective measures across "all French territories", including bolstered security at houses of worship, stores, media offices and transportation hubs.

Police officers secure a vehicle allegedly used by two gunmen in an attack on the 'Charlie Hebdo' headquarters in Paris

Police block the roads next to the 'Charly Hebdo' headquarter where a shoutout occurred in Paris

Schools across the French capital closed their doors, while top government officials held a meeting of the national emergency committee. President Hollande has scheduled a national TV address to update the public on the situation later on Wednesday evening.

Police confirmed to French media that the dead included the magazine’s editor and chief cartoonist, Stéphane Charbonnier, known as "Charb", and Jean Cabut, or "Cabu", a veteran of several French newspapers and reportedly the highest paid cartoonist in the world.

According to a journalist at the scene, at least one of the police officers killed had been assigned to protect Mr Charbonnier personally. In 2013 he was included on a "most wanted" list published by Inspire, al-Qaeda's terrorist propaganda magazine.

Charb, editor of Charlie Hebdo (Reuters) (REUTERS/Jacky Naegelen)

Jean Cabut, or 'Cabu' (AFP)

Two other cartoonists, Georges Wolinski and Bernard Verlhac, or "Tignous", were also reported as having been killed.

Charb had previously defended a controversial series of cartoons depicting the Prophet Mohammed in 2012, saying: "Muhammad isn't sacred to me. I don't blame Muslims for not laughing at our drawings. I live under French law. I don't live under Koranic law."

Charlie Hebdo's editor-in-chief Gerard Biard escaped the carnage because he was in London.

Cartoonist Georges Wolinski (AFP)

Bernard 'Tignous' Verlhac (EPA)

He told France Inter: "I don't understand how people can attack a newspaper with heavy weapons. A newspaper is not a weapon of war."

Mr Biard said he did not believe the attack was linked to the magazine's latest front page, which featured novelist Michel Houellebecq. The writer's new novel "Submission", released in France on Wednesday, has caused controversy after it was described by critics as an "Islamophobic scare story".

Yet the magazine had not received any recent threats of violence, Mr Biard said: "Not to my knowledge, and I don't think anyone had received them as individuals, because they would have talked about it. There was no particular tension at the moment."

A bullet's impact on the window of the offices of the French satirical newspaper Charlie Hebdo in Paris, after armed gunmen stormed the offices leaving at least 10 people dead

Firefighters carry an injured man on a stretcher in front of the offices of the French satirical newspaper Charlie Hebdo in Paris

A year earlier, the magazine's offices were firebombed after a spoof issue featured a caricature of Mohammed on its cover. The edition "invited" Mohammed to be its guest editor.

According to one report, journalists were discussing a new Islam-themed edition on Wednesday when the gunmen burst in.

The latest post made by the Charlie Hebdo Twitter account was a picture of the Isis militant group leader, Abu Bakr al-Baghdadi.

Jyllands-Posten, the Danish newspaper which sparked protests when it published cartoons of the Prophet Mohammed in 2005, has increased its own security in the wake of the Paris shooting, France 24 reported. Charlie Hebdo re-published the same cartoons in 2006.

Firefighters carry an injured man on a stretcher in front of the offices of the French satirical newspaper Charlie Hebdo in Paris

Police forces gather in street outside the offices in Paris

One witness to Wednesday's shooting, Gilles Boulanger, who works in the same building as the Charlie Hebdo offices, likened the scene to a war zone.

He told the BBC: "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."

French soldiers patrol in front of the Eiffel Tower in Paris as the capital was placed under the highest alert status

A police car riddled with bullets during an attack on the offices of the newspaper Charlie Hebdo in Paris

Another witness, Benoit Bringer, told TV station France Info: "We heard shouting in the street. We saw hooded men carrying Kalashnikovs enter the building. We called the police. After a few minutes we heard heavy firing, a lot of firing. We went upstairs onto the roof.

"After about 10 minutes we saw two armed men come out into the street. Three policemen arrived on bikes but had to leave because the men were armed. There was a lot more shouting in the street, a lot more gunshots. The attackers took off in a car."

David Cameron led the British response to the unfolding incident, condemning the attack on Twitter.

"The murders in Paris are sickening," he posted. "We stand with the French people in the fight against terror and defending the freedom of the press."

Philip Hammond, the British Foreign Secretary, wrote: "Appalled to hear news of apparent terrorist attack in Paris. My thoughts are with the family and friends of those killed."

Flowers, candles and a sign reading 'I am Charlie' are placed against a wall during a demonstration in Paris (AP)

Journalism students hold a banner reading in French: 'Journalism students : Solidarity' as they raise pens during a gathering at the Place de la Republique in Paris (Getty Images)

A statement from the White House also condemned the attack "in the strongest possible terms". "The US stands ready to work closely with the French," it said.

Germany's Chancellor Angela Merkel called it an "attack on freedom of speech".

"This abominable act is not only an attack on the lives of French citizens and their security," a statement read. "It is also an attack on freedom of speech and the press, core elements of our free democratic culture. In no way can this be justified."

Pope Francis has also condemned the shooting. The Vatican said it was "a double act of violence, abominable because it is both an attack against people as well as against freedom of the press".

In a statement, the French journalists' union Syndicat National des Journalistes (SNJ) said the attack represented attempts to "silence" the entire profession.

"The massacre perpetrated against the editorial staff of Charlie Hebdo is a horror that strikes at us all. When journalists are killed, it is done to make an entire profession feel fear; it is done to silence. Attacking a newspaper is an attempt to muzzle the freedom of expression that exists in a democracy."