Hundreds of people queued from the early hours outside UK shops selling the latest edition of French satirical magazine Charlie Hebdo.
It comes as around 10 people have been arrested and questioned over the "possible logistical support" they may have provided three gunmen who carried out last week's attacks in Paris, Agence France-Presse reports.
French President Francois Hollande says France will have a day of national mourning tomorrow in respect for the victims of this morning's "cowardly" Charlie Hebdo attack.
He said flags across the country will also fly at half-mast for three days as he called the victims of the massacre "heroes" who had "died for their idea of France".
"Our best weapon is our unity. Nothing should oppose us, divide us, to separate us," Hollande said.
"Freedom is always stronger than barbarism," he added.
About 7,000 French Jewish people left the country last year and have moved to Israel because they did not feel safe.
The aunt of a man killed while trying to overpower gunman Amedy Coulibaly has told ITV News her family is fearful of their future in France.
A Muslim cleric in Pakistan has held a ceremony to honour Cherif and Said Kouachi, the two men suspected of the Charlie Hebdo massacre.