Five days before the first round of France's presidential election, two men have been arrested on suspicion of planning an imminent attack.
The suspects, aged 23 and 29, were detained in Marseille by elite police and domestic intelligence agents after a search that had lasted several days.
Both men are French citizens said to have been radicalised in prison.
France remains under a state of emergency after a series of attacks that have claimed some 230 lives.
Three of the leading candidates in the presidential election - Emmanuel Macron, Marine Le Pen and François Fillon - had been warned of security risks linked to the two men, party officials said.
A flat in the rue de Crimée in Marseille's central third district was being searched, amid reports that heavy weapons and several packages of primed explosives had been found. Neither man was from Marseille.
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They were named as Mahiedine M, 29, from Croix near the Belgian border, and Clément B, aged 23 from Ermont, north of Paris.
Police said the two men were known Islamist radicals.
Interior Minister Matthias Fekl did not say what the suspects' target was.
"They had the aim of committing in the very short term, in other words in the very next days, an attack on French soil," he said.
The DGSI domestic intelligence service is said to suspect the pair of plotting an attack to coincide with the election. The men were being held as part of an inquiry into "criminal terrorist association and violating the law on arms relating to a terrorist enterprise"