It was like something out of a movie: gunfire, tyres screeching, police sirens and "rivers of blood" as terror descended on a nondescript building in a calm Parisian street.

Annick Chevalier, 50, was at work assisting the elderly near the headquarters of French satirical magazine Charlie Hebdo when she heard what she thought were fireworks.

"When I heard tyres screeching, I said to myself 'it is a film set'. But no, it was not a film. Unfortunately it was reality," she told AFP.

Minutes earlier, masked gunmen had burst into the offices of the controversial magazine where they came upon journalists holding an editorial meeting shortly after the publication of their latest edition.

"The two men opened fire and coldly executed those gathered for the editorial conference," a police source told AFP.

Outside, just up the road from the Bastille monument on a grey, finger-numbingly cold day, nursery school teacher Jean-Paul, 56, was herding a group of young children up the road after a trip to the cinema.

Suddenly he heard a ruckus from above. Looking up he saw several people leaning out of the window yelling at him to run and get the children out of there.

"We got out of there very fast. People were panicking, I heard shooting."

As he and the children scuttled off, horror unfolded inside the building as the gunmen fired off round after round, killing the editor-in-chief and four cartoonists among what would be a total of at least 12 victims.

'Bodies on ground'

Fleeing the building they shot a policeman point blank in the head, shouting "we have avenged the prophet" and "Allahu akbar" (God is greatest), according to an amateur video of the attack.

A journalist whose offices are opposite those of Charlie Hebdo described "bodies on the ground, rivers of blood and people seriously injured," in an interview with iTELE news.

Back in the street, lined with apartment buildings and a short walk from major boulevards -- where tourists and Parisians alike excitedly swarmed the streets for the first day of the winter sales -- panic and confusion broke out.

Reality quickly sunk in for Annick who peered through the second floor window of her workplace to see what was going on.

"I saw panicked police and I thought, that's it, something is happening. I saw the gangsters leave towards the Bastille in a black car. I saw them shooting, they had a long gun, I saw the fire and the smoke.

"I never would have thought I would see that ... It is atrocious, it is traumatising, it is dramatic."

The sound of sirens -- that would pierce the air for hours after the attack -- accompanied dozens of police cars, Red Cross vehicles and firetrucks that arrived on the scene.

Red and white tape went up on streets across several blocks and stunned residents struggled to come to terms with what had happened.

A portly man, who did not want to give his name, stood by, his shoulders hunched up against his blue scarf, as he watched police rush back and forth from the closed-off zone.

"I saw them leaving and shooting. They were wearing masks. These guys were serious.

"At first I thought it was special forces chasing drug traffickers or something. We weren't expecting this. You would think we were in a movie."

'They sold their soul'

A playground and several creches are dotted around the area.

Outside one nursery school with a view of the scene, teaching assistants Carole and Lucille stand smoking cigarettes and discussing what happened.

"It is shocking. Like it was a movie. Another person who works here was outside and came running in and told us to stay here with the children. We saw police and emergency workers arriving," said Carole Capulade, 24.

They huddled inside with children aged between three months and three years who were "really shaken up."

Lucille Saez, 30, said their phone was ringing off the hook as worried parents phoned to check on their children, some even stopping by to make sure everything was okay.

An outspoken moderate imam of the Mosque of Drancy, in the northern suburbs of Paris, Hassen Chalghoumi, visited the scene, calling the shooters "barbarians, they lost their soul, sold their soul to hell".

"They want terror, they want fear. We must not give in, we must all be strong. I hope the French will come out in solidarity and not against the Muslim minority in Europe."