Nervous and apprehensive as she took her daughter into Paris’s tourist centre, Ms Posetti said the city was ‘‘pretty chaotic’’, the streets filled with sirens and police whistles.

‘‘No-one was clear about what was going on but the streets were certainly busy and it wasn’t a good feeling to be in one of the world’s major tourist spots in the midst of a terrorist attack,’’ she said, adding she was angry at herself for feeling afraid.

Later, Ms Posetti – who is in Paris as a UOW research fellow with major international press freedom organisation the World Association of Newspapers and News Publishers (WAN-IFRA) – joined tens of thousands of other people at the vigil in Place de la Republique.

‘‘I was there with thousands and thousands of French citizens and other tourists mingling pretty quietly, but jam-packed, in this square,’’ she said.

‘‘I’ve never seen a crowd like it. Sure, there was fear and I imagine anxiety, but overwhelmingly there was an atmosphere of solidarity and a sense of defiance.

‘‘I watched as a man climbed the monument in Place de la Republique, which is a statue of a woman, and tied a black arm band around the statue, and the crowd was cheering ‘Charlie, Charlie’ and ‘Expression liberte’, which means freedom of expression.

‘‘There’s a sense in Paris that this is an attack, not just on a newsroom, not just an attack that has involved the murder of 12 people, but also an attack on freedom of expression which is a very central tenet or value of French society.’’

She said journalists at WAN-IFRA had labelled the attack as a broad assault on press freedom around the world.

‘‘The reality now is that it’s not safe to be a journalist anywhere, and that freedom of expression is under attack to the extent that you could be sitting in an editorial meeting in a city like Paris, in the centre of Western Europe, and find yourself gunned down because of cartoons,’’ she said.

‘‘These conversations now, about the safety of journalists and the right to report, have to be had in newsrooms around the world, not just in conflict zones.’’

France's Prime Minister says several people have been detained in the hunt for two brothers suspected of gunning down 12 people in an assault on satirical weekly Charlie Hebdo.

‘‘Several were held overnight,’’ Manuel Valls said, adding that the two suspects, who have not yet been found, were known to intelligence services and were ‘‘no doubt’’ being followed before Wednesday’s attack.

Mr Valls said preventing another attack ‘‘is our main concern,’’ as he explained why authorities released photos of the two men along with a plea for witnesses to come forward.

 Cherif Kouachi, 32, and his brother, Said, 34, are suspected of being the masked gunmen with Kalashnikov automatic weapons who entered the offices of the Charlie Hebdo newspaper at 10 Rue Nicolas-Appert in the 11th Arrondissement on Wednesday morning and slaughtered members of the paper’s staff and two police officers.

 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’’.

Interior Minister Bernard Cazeneuve said seven people had been detained and a judicial source said they were men and women close to the suspects.

The frantic man-hunt stretched into the night with search-and-seizure operations in Strasbourg and towns near Paris, while in north-eastern Reims, police commandos raided a building later scoured by  forensic police.

Le Point, a leading French newsmagazine, said  the two brothers had both been known by the intelligence services. It said that the police had identified the suspects after one left his identification papers in the abandoned get-away car.

The massacre, which singled out cartoonists and other staff members at a newspaper that frequently mocked Islam, Christianity and all forms of religious and secular authority, left France stunned. It also raised questions about how Cherif Kouachi, so well-known to the police for so many years, and his brother had managed to conceal their intentions.

 President Francois Hollande ordered flags to fly at half-mast for three days  and was due to convene an emergency cabinet meeting at 8.30am local time on Thursday.

A minute’s silence will be observed across the country at midday, after which the bells of Paris’ famous Notre Dame cathedral will sound out across the capital.

‘‘Nothing can divide us, nothing should separate us. Freedom will always be stronger than barbarity,’’ said the president, calling for ‘‘national unity’’.

Even before the attack, France, home to Europe’s biggest Muslim population, was on high alert like many countries in which citizens have left to fight alongside the radical Islamic State group in Iraq and Syria.

‘‘Several terrorist attacks had been foiled in recent weeks,’’ Mr Hollande said.

About 11.30am 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.

Outside the building, chilling amateur video footage showed the attackers calmly approaching a wounded policeman a lying  on the pavement and then shooting him at close range.

 Some witnesses described ‘‘rivers of blood’’ flowing in the streets of the City of Light.    

AFP/AP