PARIS, France (AFP) — Seven Frenchmen arrested as part of a weekend swoop on a suspected Islamist terror cell believed to have plotted attacks in the country were formally charged yesterday, a legal source said.

An alleged leading member of the group, Jeremy Bailly, was the first to be charged and detained on suspicion of having organised a grenade attack on a Jewish grocery store in a Paris suburb last month.

A French prosecutor yesterday branded the suspected homegrown group of Islamist extremists as the biggest terror threat the country has faced since the Algerian-based GIA carried out a string of deadly bombings in the 1990s.

Announcing charges of attempted murder and terrorism against seven out of 12 suspects arrested over the weekend, Paris prosecutor Francois Molins said they had been part of an active terror cell that was "probably the most dangerous in France since 1996."

The group had been plotting to mount attacks in France and to join up with other "jihadists" in Syria and elsewhere, Molins said. The profile of the suspects in custody was "much more dangerous than we initially assumed", he added.

"We have been able to avoid a terrorist attack in our country," Molins said.

Later yesterday, French President Francois Hollande vowed to track down and destroy Islamist extremist cells operating inside the country.

"We will go after them and we will eliminate them," he told French broadcasters.

Hollande also argued that letting extremism flourish abroad could come back to hurt France.

Authorities had information there were French nationals with the hardline Islamists in northern Mali, Somalia and Syria, he said. He could not permit them to return to France with plans to launch attacks there.

The grenade attack on a Jewish grocery store in the Paris suburb of Sarcelles last month left one person slightly injured but Molins said the grenade could have seriously injured anyone within a 10-metre radius.

Police have since discovered weapons and significant amounts of bomb-making equipment at the home of Bailly.

Bailly has admitted attempting to fabricate a bomb but has refused to say whether anyone else was involved or what his potential target was.

Five of the 12 initially detained were released without charge yesterday.

The other seven are all French citizens, aged between 19 and 25, Molins said. All but one of them are converts to Islam. Two of them had served time in prison for drug dealing.