French police said Wednesday they had seized bomb-making equipment from the homes of a group of Islamic extremists they now believe constituted an "extremely dangerous terrorist cell."

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Francois Molins, the Paris prosecutor heading an investigation triggered by a grenade attack on a Jewish grocery store, invoked rarely-used anti-terrorist legislation to extend the detention of 12 suspects into a fifth day.

The French criminal code allows for suspects to be held without charge for up to six days in cases of a "serious risk of an imminent terrorist attack" in France or abroad.

"We are clearly and objectively facing an extremely dangerous terrorist cell," Molins said, defending the extraordinary detentions as necessary to "avoid the risk of a terrorist attack in France".

He said "components useful for bomb-making", a shotgun and a handgun had been found in searches in the eastern Paris suburb of Torcy, where two of the suspects were detained on Saturday.

Among the components found were bags of potassium nitrate, sulphur, saltpetre, pressure cookers and headlight bulbs, "all products or instruments useful in the making of what we call improvised explosives," Molins said.

Police spent most of Wednesday removing sacks of unidentified material from an underground garage at an apartment block in Torcy.

The 12 alleged members of the cell are all under 30 and thought to have been either born or brought up in France.

They are being held held on suspicion of involvement in a grenade attack on a Jewish grocery store in the Paris suburb of Sarcelles last month and of planning other anti-Semitic attacks.

A list of Jewish organisations in the Paris area was found at one of the addresses where the bomb-making components were discovered.

The suspected leader of those detained, 33-year-old Jeremie Louis-Sidney, was shot dead Saturday after he opened fire on officers seeking to arrest him in a dawn raid at his home in Strasbourg.

Police were led to Louis-Sidney, a convert to Islam who was radicalised during a spell in prison for drug dealing, following forensic examination of the pin of a grenade thrown into the kosher grocery on September 19th.

Traces found on the pin suggested he had handled the grenade but Molins said it was not clear if he had thrown it and said two men believed to have been directly involved in the attack may still be at large.

"It has not yet been established that the two individuals who carried out the attack by throwing the grenade into the grocery have been apprehended," he said.

Wednesday's decision to continue to hold the suspects without charge beyond four days marks only the second time such an extension has been granted since France's current pre-charge detention system was adopted in 2006.

Sources told AFP the suspects were all refusing to cooperate during interrogation by anti-terrorism officers.

"In the three weeks leading up to the arrests, physical and telephone surveillance of the members of the group showed that they were all very active, mobile and extremely prudent about their movements," the source added.