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.
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".
'A significant threat': Hugh Schofield, BBC News, Paris
It is a working assumption of the French intelligence services that Islamist extremists would like to target the elections.
The line of reasoning goes like this: if there is a lethal attack, then people are more likely to vote for the far-right - and that is what the jihadists want, because a victory for Marine Le Pen could tip the country into chaos.
Much remains to be told about the pair who have been arrested in Marseille. We do not know who or what it was they were allegedly planning to attack. We do not know how close they were to action. We do not know if they were guided by figures from abroad.
But until evidence suggests otherwise, their enterprise will be treated as a significant and genuine threat.
With France approaching the climax of the election campaign, the arrests became a major political development on Tuesday.
It has emerged that centre-right candidate François Fillon was warned of "confirmed risks" late last week, with the focus said to be an Easter rally in Nice.
A Fillon spokesman told French media that since Thursday "the security detail was reinforced around him during his travels".
Photos of the two suspects were also circulated last week to security officers looking after the two election front-runners: centrist Emmanuel Macron and far-right leader Marine Le Pen.
Mr Macron called for unity and said the arrests were a reminder of the "strong terrorist threat" facing France.
Ms Le Pen has accused the government of failing to tackle militant Islamists and her adviser, Florian Philippot, said on French TV that "our democracy itself, our voting operation and electoral campaigns are under threat".
Far-left candidate Jean-Luc Mélenchon, himself riding high in the opinion polls, sent text messages of support to Mr Fillon and Mr Macron offering his "complete solidarity".
Eleven candidates will contest the first round on 23 April, with the top two qualifying for the run-off on 7 May.