Police said they found the explosives in the 3rd-floor menswear department bathroom of the Printemps store on Paris' busy Boulevard Haussman after receiving a warning from a previously unknown group called the Afghan Revolutionary Front, The Times of London reported.

The letter, calling for the withdrawal of French troops from Afghanistan by February, said the dynamite would explode if not defused, but police said there was no detonation system to trigger the explosives and called the device "amateurish."

"As far as we can tell, the system was not destined to explode. We are going to find the authors," the French Interior Minister Michelle Alliot-Marie told reporters after meeting with police at the scene.

The Printemps store was evacuated and Boulevard Haussman closed to traffic, causing huge jams in central Paris, The Times reported.