One minute's silence was observed all over the country on Thursday; bells of Notre Dame rang at midday and flags fly at half-mast for three days in memory of the victims of Charlie Hebdo shooting in Paris.

In a prime-time TV evening address, French President Francois Hollande had declared Thursday a mourning day, calling all French citizens to unite together at this difficult time in France.

"Our best weapon is our unity. Nothing can divide us, nothing can stop us, and nothing can separate us...Liberty will always be stronger than barbarity," he said.

On Wednesday, two masked and heavily armed men stormed offices of the satirical magazine Charlie Hebdo and killed 12 people.

In November 2011, the magazine's headquarters was firebombed after it put an image of the Prophet Mohammad on its cover.

In its last published cartoons, the weekly mocked Abu Bakr al-Baghdadi, the leader of the Islamic State, which seized majot towns in Iraq and Syria.

Identified as Cherif Kouachi, 32, and his 34-year-old brother Said, the two suspects from Paris region are still at large after 24 hours of the shooting. Reports said they were located in the Aisne in north eastern France.

A nationwide manhunt is widened to find the "armed and dangerous men".

According to some reports, Cherif Kouachi had previously been trialed on terrorism charges and served 18 months in prison.

He was charged with criminal association related to a terrorist cell in 2005. He had been part of an Islamist cell that enlisted French nationals from a mosque in eastern Paris to go to Iraq to fight Americans in Iraq. He was arrested before leaving for Iraq to join militants.

Earlier on Thursday, a policewoman succumbed to her injuries after a gunman shot at her and a civil employee before fleeing in Montrouge, a neighbouring city in southern Paris.

It's not still clear whether the shoutout in southern Paris has links with Charile Hebdo shooting.

France's security alert remains at the highest level on fears of eventual attacks with security measures tightened at transport hubs, religious sites, media offices and department stores.

Wednesday's shooting, the country's worst attack since the second World War, posed tests to the government's security and raised fears of anti-Islam feelings in France where 5 million Muslims are living, the largest community in Europe.

Local media reported an overnight explosion hit a restaurant next to a mosque in the central town of Villefrance-sur-Saone. Another attack against a Muslim place of worship was also reported in Mans in western France.