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.
David Cameron says future legislation will be necessary to ensure authorities have access to "vital" communications data that could help prevent terror attacks and find missing people.
Speaking in Nottingham, he said the legislation should also allow the content of such messages - sent via the internet - to be made available to the government "in extremis".
He added that if he remained Prime Minister after the General Election he would ensure "comprehensive" legislation was passed so that Britain did not "allow terrorists safe space to communicate with each other".
"The first duty of any government is to keep our country and our people safe," Cameron said.
"The attacks in Paris once again demonstrated the scale of the terrorist threat that we face and the need to have robust powers - through our intelligence and security agencies and policing - in order to keep our people safe."
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.