Police reinforcements will be deployed in Paris and other big French cities after the discovery of dynamite in a top Paris department store on Tuesday.
Interior Minister Michele Alliot-Marie announced a meeting of police, intelligence and transport chiefs and said security arrangements would be reinforced.
Police found five sticks of explosives in a third-floor toilet of the store, Printemps Haussmann, though the dynamite was not equipped with a detonator.
The department store on the capital's best-known shopping boulevard was cordoned off while hordes of Christmas shoppers and tourists were evacuated. It reopened in the afternoon.
An unknown group calling itself the Afghan Revolutionary Front had earlier sent a letter to news agency Agence France-Presse warning of "several bombs" in the upmarket store.
Security in other French cities was stepped up in early December ahead of the busy Christmas season, with extra groups of police patrolling the Champs Elysees avenue and other top shopping districts.