Standing in support of the freedom of the press, these protesters hold an illuminated sign at a gathering in the centre of Paris. Photo: AFP

French security forces desperately hunted two brothers suspected of gunning down 12 people in an Islamist attack on a satirical weekly, as a stunned and outraged France fell silent to mourn the victims yesterday.

Bells pealed out across France at the stroke of midday, public transport stopped and people gathered outside the headquarters of the Charlie Hebdo magazine in the pouring rain, holding aloft banners reading "Je Suis Charlie" (I am Charlie).

Television pictures showed children at a Muslim school in the northern city of Lille holding up sheets of paper with "not in my name".

The two brothers -- Cherif Kouachi, 32, and Said, 34, -- were apparently spotted at a petrol station in the northern Aisne region yesterday.

French investigators found a dozen Molotov cocktails and two jihadist flags in the getaway car used in the massacre on Wednesday, a source close to the case told AFP.

"This shows their Islamist radicalisation and that they had possibly planned other acts with the petrol bombs," the source said.

Security forces deployed in a northern town where two brothers – still armed -- abandoned their car, a police source said.

The suspects are said to have robbed a service station in the north of France. They stole food and petrol, firing shots as they struck at the roadside stop near Villers-Cotterets in the Aisne region, French media report.

Wednesday's massacre triggered poignant and spontaneous demonstrations of solidarity around the world and more than 100,000 poured onto the streets of France.

As fear spread in France, several other incidents rocked the jittery nation, although it was not clear whether they were linked to Wednesday's attack.

A gunman shot dead a policewoman and wounded a city employee with an automatic rifle just to the south of Paris and there was an explosion at a kebab shop in eastern France, with no casualties immediately reported.

It is unclear if the attack on the policewoman is related to the pursuit of prime suspects Cherif and Said Kouachi, reports BBC.

Two Muslim places of worship were also fired at in the wake of Wednesday's attacks, prosecutors said.

Declaring yesterday a national day of mourning -- only the fifth in the last 50 years -- President Francois Hollande called the bloodbath "an act of exceptional barbarity".

France has observed a minute's silence for the 12 people killed at the office of the satirical magazine.

But more than 24 hours after the brazen assault, the masked, black-clad gunmen -- who shouted "Allahu Akbar" ("God is greatest") while killing some of France's most outspoken journalists as well as two policemen -- were still on the loose.

Police issued arrest warrants for Cherif, a known jihadist convicted in 2008 for involvement in a network sending fighters to Iraq, and Said. Both were born in Paris.

Interior Minister Bernard Cazeneuve said seven people had been detained in the hunt for the brothers, and a judicial source added that the men and women held were close to the suspects.

Prime Minister Manuel Valls, meanwhile, told French radio the two suspects were known to intelligence services and were "no doubt" being followed before Wednesday's attack.

The manhunt stretched into the night with search-and-seizure operations in Strasbourg and towns near Paris, while in northeastern Reims, police commandos raided a building later scoured by white-clad forensic police.

Hamyd Mourad, an 18-year-old suspected of being an accomplice in the attack, handed himself in, with police sources saying he had seen his name "circulating on social media".

France's main Islamic groups urged Muslims across the country to observe the minute of silence and for imams to condemn terrorism.

At around 11:30 am on Wednesday, the killers stormed the central Paris offices of Charlie Hebdo during an editorial meeting and picked off some of France's best-known cartoonists in cold, military-style executions. Many witnesses said the scene was "like a movie".

Charlie Hebdo will come out next week despite the decimation of its staff, one of the magazine's columnists said.