(Updated Thursday 2pm)

French authorities are hunting two heavily armed brothers they fear could strike again after the killing of 12 people at a satirical newspaper in Paris that caricatured the Prophet Mohammed.

Prime minister Manuel Valls said the possibility of a new attack "is our main concern" and announced several overnight arrests as the country began a day of national mourning.

The most senior security official abandoned a top-level meeting today after just 10 minutes to rush to a shooting on the city's southern edge that killed a policewoman.

The shooter remained at large and it was not immediately clear if her death was linked to yesterday's deadly attack on the Paris newspaper Charlie Hebdo, where two police were among the 12 dead.

Mr Valls said the two suspects still at large in the Charlie Hebdo killings - Cherif Kouachi, 32, and his 34-year-old brother Said Kouachi - were known to intelligence services and preventing them from carrying out another attack "is our main concern".

Police swarmed a petrol station in the northern Aisne region where the two men were reportedly spotted early today and helicopters hovered above the site.

Mr Valls told RTL radio several people had been arrested overnight; a security official put the total at seven. A third suspect in the Charlie Hebdo killings has already turned himself in.

Fears have run high in Europe that jihadis trained in warfare abroad would stage attacks at home. The French suspect in a deadly 2014 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 midway between France's Bastille and the city's enormous Republique plaza.

At noon local time, the Paris metro came to a standstill and people gathered in homage near Notre Dame Cathedral fell silent to honour Wednesday's victims.

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

A third man, Mourad Hamyd, 18, surrendered at an eastern police station after learning his name was linked to the attacks in the news, said the Paris prosecutor 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 guarding media offices, places of worship, transport and other sensitive areas.

Fears arose that the deadly attack would ignite a backlash against France's large and diverse Muslim community. A mosque in the city of Le Mans, 200 kilometres (125 miles) south west of Paris, was hit overnight with training grenades and gunfire, local prosecutor Philippe Varin told the Associated Press. No one was in the building at the time and no one was injured.

One witness to yesterday'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 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 then called out the names of specific employees.

Eight journalists, two police officers, a maintenance worker and a visitor were killed and 11 people were wounded, four of them critically.

Two gunmen strolled out to a black car waiting below, one of them calmly shooting a wounded police officer in the head, according to video and a nearby witness.

"They knew exactly what they had to do and exactly where to shoot," said the witness.

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."

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. A caricature of Islamic State's leader was the last tweet sent out by the newspaper, minutes before the attack. Its feed has since gone silent.

Cherif Kouachi, who was sentenced to 18 months in prison after being convicted of terrorism charges in 2008, has said he was outraged at the torture of Iraqi inmates at the US prison at Abu Ghraib near Baghdad.

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