PRIME Minister David Cameron will have a "full intelligence briefing" on the Paris attack from security agencies later today, and stands ready to offer any assistance needed by France, Downing Street said.

Twelve people were shot dead in a terror attack at the offices of a French satirical weekly which angered some Muslims after publishing crude caricatures of Islam's prophet Mohammed.

Masked gunmen stormed the headquarters of Charlie Hebdo, shooting indiscriminately.

The gunmen, who later fled, were heard shouting "Allahu Akbar" - God is greatest - as they opened fire.

Video footage taken by terrified witnesses from windows and on rooftops overlooking the scene showed the terrorists shooting one of their victims, who appears to be in a police uniform, in cold blood at close range as he lay already injured on a pavement of the otherwise deserted Paris street.

French president Francois Hollande said the "terrorist attack" had left France in a state of shock.

He said: "We are looking for the perpetrators of this crime.

"France is today in shock, in front of a terrorist attack.

"This newspaper was threatened several times in the past. We need to show that we are a united country. We have to be firm, we have to be strong.

The Northern Echo:

"We are at a very difficult moment. Several terrorist attacks have been impeded during the previous weeks. We are threatened because we are a country of freedom.

"We fight threats and we will punish the attackers."

Mr Cameron joined the condemnation of the attack, saying: "The murders in Paris are sickening. We stand with the French people in the fight against terror and defending the freedom of the press."

Charlie Hebdo has launched a series of attacks on Muslim extremism and the last tweet on its profile page @Charlie_Hebdo_, sent about an hour before the shootings, included a satirical cartoon of Islamic State leader Abu Bakr al-Baghdadi.

In it he wishes everyone "good health".

The attackers, who fled in a dark-coloured hatchback, reportedly shouted "We have avenged the prophet".

Witness David Chour told the BBC from close to the scene: "A lot of people from the shops heard a lot of gunshots.

"Two guys entered with Kalashnikov guns and shot the people. I haven't seen them directly but people around said they just went away.

"People are very shocked."

As the manhunt for the attackers began, the massacre prompted the French government to raise the national security alert system to "alerte attentat", the highest level, across the entire Ile-de-France region around Paris.

The shootings in the capital come shortly after an outbreak of opportunist attacks in crowded spaces across France.

Late last month, a van burst into a Christmas market in the western city of Nantes, injuring 10 people before the driver reportedly began stabbing himself.

In the eastern city of Dijon, a driver reportedly shouting ''God is great'' in Arabic ran down several people, injuring 13 before coming to a stop.

Those incidents came after an attacker knifed three police officers in Tours before he was shot dead by one of the officers.

The French government denied links between the attacks but announced plans to further raise security in public spaces, including the deployment of around 300 soldiers.

Repeated shots could be heard in video footage of today's attack filmed by witnesses from various vantage points.

One clip shows what appears to be a police officer being shot at close range, before two masked men, dressed in black, get into a car.

In another clip, a passer-by hearing the attack dives for cover between two cars.

Witnesses at the magazine headquarters described a scene of carnage, with bullet holes and smashed windows.

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."

Local news outlet Le Point has reported that cartoonists Jean Cabu, Stephane Charbonnier and Bernard "Tignous" Verlhac are among the dead.

Charlie Hebdo's website lists "Charb" as its publication director, and "Cabu" as artistic director.

Mr Charbonnier was included in a 2013 Wanted Dead or Alive for Crimes Against Islam article published by Inspire, the terrorist propaganda magazine published by al Qaida.

Charlie Hebdo's editor-in-chief Gerard Biard has told France Inter: "I don't understand how people can attack a newspaper with heavy weapons. A newspaper is not a weapon of war."

Ian Hislop, editor of British satirical magazine Private Eye, said the victims of the Charlie Hebdo shooting "paid a very high price for exercising their comic liberty".

The dead are believed to include editorial staff members Jean Cabu, Stephane "Charb" Charbonnier, Bernard "Tignous" Verlhac and Bernard Maris.

In a statement, journalist and writer Mr Hislop said: "I am appalled and shocked by this horrific attack - a murderous attack on free speech in the heart of Europe.

"I offer my condolences to the families and friends of those killed - the cartoonists, journalists and those who were trying to protect them.

"They paid a very high price for exercising their comic liberty."

He added: "Very little seems funny today."