"You can imagine what their other plans could have been," counterterrorism official Eric Voulleminot said at a news conference with Molins.
The prosecutor described 33-year-old Sydney, sentenced in 2008 to two years in prison for drug trafficking, as a "delinquent who converted to radical Islam." He said others in the cell indicated they wanted to return to "the land of jihad."
A statement from President Francois Hollande praised the police for the raids and said the state would continue to "protect the French against all terrorist threats."
Last month's firebombing of the grocery, in a Jewish neighborhood in the Paris suburb of Sarcelles, happened on Sept. 19, the same day a French satirical paper published crude caricatures of the Prophet Muhammad. Anti-Western protests were also growing at the time against an anti-Islam film. One person was slightly injured, but the attack with a Yugoslav grenade came after a summer of what residents described as growing anti-Semitic threats.
"What happened in Sarcelles was just a start, or was just a test," Sammy Ghozlan, head of a French group that tracks anti-Semitism in the country, said. "Islamism is a force of influence and Islamists are going to seek out the weakest people to teach them to kill."
France, which has the largest Muslim population in Europe, is trying to contain the spread of a radical Islam hostile to Western influences. France has made similar anti-terrorism arrests before, only to release the suspects several days later without charges.
The prosecutor was careful not to draw direct links between Saturday's arrests and Mohamed Merah, a young Frenchman of Algerian descent who died in a shootout with police in March after the killings in the south of France. That attack terrorized the French Jewish community, which has since ramped up security in many parts of the country.
Merah had studied at an Islamist paramilitary camp in Pakistan and claimed ties to al-Qaida. Molins said officials did not believe the men arrested Saturday had trained abroad, but cautioned that the investigation was ongoing.