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
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 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.
The interior minister said 50,000 members of the security forces were being deployed for the elections, particularly at polling stations.
President François Hollande praised the Marseille operation, which began after 10:00 (08:00 GMT), as a "remarkable capture".
Although no major attack has been carried out on French soil in recent months, two soldiers were attacked with machetes outside the Louvre museum in Paris in February. In the same month, three men and a 16-year-old girl were detained in Montpellier on suspicion of planning a bomb attack.
France has been under a state of emergency since the Paris attacks in November 2015 and officials say the risk is still very high, pointing to recent murders in the UK and Sweden.
On 22 March, Khalid Masood killed five people in London when he drove into pedestrians on Westminster Bridge and stabbed PC Keith Palmer outside Parliament.
On 7 April, Rakhmat Akilov, 39, drove a lorry into a Stockholm department store, leaving four people dead.