French anti-terror police have found bomb-making material and weapons while investigating suspected radical Islamists across France and believe they have uncovered a terror network, Paris prosecutor Francois Molins said on Wednesday.

Molins said that a dozen suspects rounded up in a raid over the weekend would be held for a further 24 hours after the discovery of chemicals used to make explosives at a garage in the Paris suburbs belonging to one of them.

"We are clearly confronted with an extremely dangerous terror network," Molins said in a statement to the media. "It is essential to extend their stay in custody."

On Saturday, French police carried out raids across France, targeting a suspected jihadist cell of young Frenchmen recently converted to Islam. DNA on a grenade that exploded last month at a kosher grocery store led them to a member of the cell, who was killed in a shootout with police Saturday.

Officials said he had been under surveillance since last spring — around the time a French Islamist radical went on a shooting rampage against a Jewish school and French soldiers, killing seven people.

A leading French Muslim organization, the CFCM, denounced the synagogue attack. It said in a statement Sunday that it "assures the French Jewish community of its support and fraternal solidarity in the face of all attacks that target its members and institutions."

On Sunday, French President Francois Hollande met with leaders of the country's Jewish community and pledged to fight extremism and anti-Semitism "with the greatest firmness.

He said that authorities "in the coming days, in the coming hours" will increase security at Jewish religious sites so they won't be subject to the kind of attack that targeted a synagogue in the Paris suburb of Argenteuil on Saturday night.