A policewoman shot in an early morning attack on the southern edge of Paris has died, a French police official said.

The officer was stopping to investigate a traffic accident when an assailant opened fire, hitting her and a street sweeper. Emmanuel Cravello, of the Alliance police union, said she later died of her injuries.

French interior minister Bernard Cazeneuve said the attacker in today's shooting remained at large.

He and Mr Cravello cautioned against jumping to any conclusions about the attack, which has not been linked to yesterday's assault on the newspaper Charlie Hebdo, which left two police officers among the dead.

French police are hunting for two heavily armed men - one with a terrorism conviction and a history in jihadi networks - over the methodical killing of 12 people at the satirical newspaper that caricatured the Prophet Mohammed.

Prime minister Manuel Valls announced several overnight arrests and said the possibility of a new attack "is our main concern".

Tensions in Paris were already high as France began a day of national mourning.

It was not immediately clear if today's shooting was linked to the Charlie Hebdo attack.

Mr Valls said the two suspects in the Charlie Hebdo shootings were known to intelligence services, and the fear that they could carry out another attack "is our main concern".

He told RTL radio there had been several arrests overnight; a security official put the arrest total at seven.

Mr Valls said it is likely the suspects were being tracked by intelligence services but "there is no such thing as zero risk".

Fears have run high in Europe that jihadis trained in warfare abroad would stage attacks at home. The French suspect in a deadly attack on a Jewish museum in Belgium had returned from fighting with extremists in Syria; and the man who rampaged in the south of France in 2012, killing three soldiers and four people at a Jewish school in Toulouse, received paramilitary training in Pakistan.

"France has been struck directly in the heart of its capital, in a place where the spirit of liberty - and thus of resistance - breathed freely," President Francois Hollande said. The attack took place nearly midway between France's Bastille and the enormous Republique plaza.

One of the Charlie Hebdo suspects, Cherif Kouachi, was convicted of terrorism in 2008 for involvement in a network sending radical fighters to Iraq. He and his brother, Said, should be considered "armed and dangerous", French police said in a bulletin, appealing for witnesses after a fruitless search in the city of Reims, in French Champagne country.

A third man, Mourad Hamyd, 18, surrendered at a police station in a small town in the eastern region after learning his name was linked to the attacks in the news and social media, said Paris prosecutor's spokeswoman Agnes Thibault-Lecuivre. She did not specify his relationship to the Kouachi brothers.

France raised its terror alert system to the maximum and bolstered security with more than 800 extra soldiers to guard media offices, places of worship, transport and other sensitive areas. A nationwide minute of silence was being held at noon.

One witness to Wednesday's attack said the gunmen were so methodical he at first mistook them for an elite anti-terrorism squad. Then they fired on a police officer.

The masked, black-clad men with assault rifles launched the attack at around noon. The publication had long drawn condemnation and threats - it was firebombed in 2011 - for its depictions of Islam, although it also satirised other religions and political figures.

The staff were in an editorial meeting and the gunmen headed straight for the paper's editor, Stephane Charbonnier, widely known by his pen name Charb, killing him and his police bodyguard first, said Christophe Crepin, a police union spokesman.

Shouting "Allahu akbar!" as they fired, the men spoke in fluent, unaccented French as they called out the names of specific employees.

Eight journalists, two police officers, a maintenance worker and a visitor were killed, said prosecutor Francois Molins. He said 11 people were wounded, four of them seriously.

Two gunmen strolled out to a black car waiting below, one of them calmly shooting a wounded police officer in the head as he writhed on the ground, according to video and a man who watched in fear from his home across the street.

"They knew exactly what they had to do and exactly where to shoot. While one kept watch and checked that the traffic was good for them, the other one delivered the final coup de grace," said the witness.

"Hey! We avenged the Prophet Muhammad! We killed Charlie Hebdo," one of the men shouted in French, according to video shot from a nearby building.

One police official said the suspects were linked to a Yemeni terrorist network. Cedric Le Bechec, a witness who encountered the escaping gunmen, quoted the attackers as saying: "You can tell the media that it's al Qaida in Yemen."

After fleeing, the attackers collided with another vehicle, then hijacked another car before disappearing in broad daylight, Molins said.

Among the other dead were cartoonists Georges Wolinski and Berbard Verlhac, better known as Tignous, and Jean Cabut, known as Cabu. Also killed was Bernard Maris, an economist who was a contributor to the newspaper and was heard regularly on French radio.

One cartoon, released in this week's issue and titled Still No Attacks in France, had a caricature of a jihadi fighter saying "Just wait - we have until the end of January to present our New Year's wishes." Charb was the artist.

In a sombre address to the nation last night, Mr Hollande pledged to hunt down the killers, and pleaded with his compatriots to come together in a time of insecurity and suspicion.

"Let us unite, and we will win," he said. "Vive la France!"

Thousands of people later jammed the Republique plaza to honour the victims, waving pens and papers reading "Je suis Charlie" - "I am Charlie." Similar rallies were held in London's Trafalgar Square as well as Madrid, Barcelona, Berlin and Brussels.

"This is the darkest day of the history of the French press," said Christophe DeLoire, of Reporters Without Borders.

Both al Qaida and the Islamic State group have repeatedly threatened to attack France, which is conducting air strikes against extremists in Iraq and fighting Islamic militants in Africa. Charb was specifically threatened in a 2013 edition of the al Qaida magazine Inspire, which also included an article titled France the Imbecile Invader.

Cherif Kouachi, now 32, was sentenced to 18 months in prison after being convicted of terrorism charges in 2008 for helping funnel fighters to Iraq's insurgency. He said he was outraged at the torture of Iraqi inmates at the US prison at Abu Ghraib near Baghdad and "really believed in the idea" of fighting the US-led coalition in Iraq.

A tweet from an al Qaida representative who communicated on Wednesday with the Associated Press said the group was not claiming responsibility for the attack, but called it "inspiring".