French anti-terrorist police have found bomb-making materials and weapons while investigating suspected radical Islamists across the country.
Authorities there say they have rooted out an "extremely dangerous terror network".
Paris prosecutor Francois Molins said a dozen people arrested on Saturday would be held for a further 24 hours, after chemicals used to make explosives were found at a garage in a Paris suburb belonging to one of them.
The raids were triggered by a grenade attack on a kosher grocery store outside Paris last month.
Saturday's raid ended with police shooting dead an Islamist suspect linked to the grenade attack and arresting 11 others.
Another suspect was subsequently arrested.
Police at the weekend found a list of local Jewish groups at the home of one of the suspects.
Mr Molins said a search of the suspect's garage had revealed a shotgun, a revolver, bags of potassium nitrate, sulphur and a pressure cooker.
"These are all products used to make what we call improvised explosives," he said.
France's Jewish community has been on edge after a series of attacks in recent months.
In the worst incident, three Jewish children and a rabbi were among seven shot dead in the southern city of Toulouse in March by an al-Qaeda-inspired gunman.
Last week, the French government said that the terrorist threat remained high.
It presented legislation allowing police to arrest those believed to have been involved in terrorism-related activity outside French borders.