Two men have been arrested on suspicion of planning an “imminent” attack in France, days before the first round of the French presidential election.
Police said they found explosives and several guns at a flat linked to the suspects.
The two Frenchmen, aged 23 and 29, were detained in Marseille on Tuesday. They were reportedly radicalised in prison.
France remains under a state of emergency after a series of attacks that have claimed about 230 lives.
Three of the leading candidates in the presidential election – Emmanuel Macron, Marine Le Pen and François Fillon – were warned last week of security risks linked to the two men, party officials said.
The suspects are Mahiedine M, 29, from Croix near the Belgian border, and Clément B, aged 23 from Ermont, north of Paris.
The two men had met in prison, and were known to police as Islamist radicals.
French prosecutor François Molins said they had been planning an “imminent” attack but the target, or targets, remained unknown.
He said they had made three kilograms of the highly volatile explosive TATP, with one batch ready for use.
Mahiedine M had made a video featuring the black flag of the so-called Islamic State group, an Uzi sub-machine gun, and a newspaper depicting a presidential candidate, Mr Molins said.