PARIS (AFP) - France observed a minute of silence Thursday, broken only by church bells, in honour of the 12 people killed by extremists at a magazine known for publishing cartoons deemed offensive to Islam.

At midday (7pm Singapore time), crowds of people stood silently in public squares, schools and outside official buildings. Bells tolled at Paris' Notre Dame cathedral and in churches across the country.

Office workers stood shoulder to shoulder, buses and metros halted, and only the toll of bells and sound of weeping broke the silence on Thursday as France honoured the 12 people massacred at Charlie Hebdo magazine.

"Charlie will be free!" cried a woman in the a large crowd in front of the medieval Notre Dame cathedral.

Among the hundreds gathered on the ancient square, many were in tears or stood with their eyes closed, while some prayed and a long line formed to enter the cathedral for a special memorial Mass.

Across the city, at the major rail station of Saint-Lazare, staff called on travellers and workers to pause at midday. "We must stick together and save our freedom of speech," said Julie, 37, who works for the national SNCF rail company.

Another Paris icon, the Eiffel Tower, was to dim its lights at 8:00 pm (3am Friday Singapore time).

Ten people at Charlie Hebdo - including the chief editor and renowned cartoonists - were gunned down Wednesday by two men who shouted they were taking revenge for the magazine's repeated publication of cartoons widely seen as insulting to Islam. Two policemen were also shot, one of them finished off at close range as he lay wounded on the sidewalk.

Shocked politicians led by President Francois Hollande were seen on television taking part in the minute of silence.

Sorrow and fear spread right through a country that has long prided itself on freedom of expression, but which for decades has struggled to integrate its rapidly growing Muslim population.

In Bordeaux, capital of France's most famous wine growing region, mourners gathered late into the night and continued to come by early Thursday leaving candles, flowers, inscriptions of support and old copies of Charlie Hebdo at a makeshift memorial.

In Nantes, in western France, a young man at a similar memorial was in tears, bearing the words "Je suis Charlie" or "I am Charlie" on his black T-shirt.

The phrase has gone viral at impromptu demonstrations and in social media campaigns over the last 24 hours, even featuring at a demonstration of several hundred people on the French island of La Reunion in the Indian Ocean, some 9,000 kilometres away.

"They wanted to kill Charlie Hebdo, but they made it immortal," the man in Nantes said.

CHILDREN'S FEARS

As France tried to come to terms with the bloodbath in what had been a quiet Paris neighbourhood, parents wondered what to tell their children.

In Saint-Germain-en-Laye, just outside Paris, one mother said she explained what had happened to her children before they heard about it in the playground, "where things could get twisted." Another mother, though, said she "couldn't find the words. I hope the teacher will do it."

In the neighbourhood where the Charlie Hebdo offices are located, Herve Roch, the father of two, said he'd told his children, "that evil people came to do bad things and the police would catch them."

Sarah, 12, said she did not want to go alone to school. Her mother decided to accompany her. "It's important that she goes. If we allow ourselves to be afraid, they will win," she said.