Terrorism burst into the final week of the French presidential campaign after an "imminent" plot to attack a candidate was foiled, two men arrested and guns and explosives seized - reportedly thanks to a tip-off from British intelligence.

With five days to go before the first round of the most unpredictable presidential race in modern French history, agents and anti-terror police seized two suspects, Clément Baur, 23 and Mahiedine Merabet, 29, in Marseille, southern France.

The pair, one from Croix in northern France, the from the Val d'Oise near Paris, were arrested a few minutes apart in the 3rd arrondissement of the port city.

A black Isis flag, several weapons - including an Uzi submachine gun - and three kilogrammes of TATP-type explosives used in the Paris and Brussels suicide bombings were found in a subsequent raid, said Francis Molins, the Paris prosecutor.

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Some of the bomb material was "ready for use", he added.

President François Hollande hailed the arrests as a "remarkable haul".

Matthias Fekl, the French Interior Minister, said the two French nationals were "suspected of wanting to launch an imminent violent act on the eve of the presidential elections".

He added: "They intended to commit an attack in the very short term, namely in the coming days on French soil."

Molins said the two men appeared to have turned to radical Islam during a two-month spell sharing a cell in a northern French prison.

Describing them as "wary and determined", he said police were unsure about "the precise day" of the planned strike and of "the target or targets".

But according to Le Monde, a video police seized of one suspect shows him standing before an Isis flag holding a machine gun and a copy of daily newspaper with François Fillon, the conservative presidential candidate, on the cover, with the words "la loi du talion (an eye for an eye)" on the screen.

Fillon's team was warned late last week of a "confirmed risk" of attack and shown a photo of the two suspects, said French media. Le Figaro cited a source close to Fillon as saying: "The initial intelligence came from British services who are thought to have intercepted data from two individuals known to security services."

The centre-right contender held rallies in Montpellier last Saturday and yesterday in Nice, the scene of a deadly Isis truck attack that killed 86 last July. Anti-terror police and snipers were present at the meetings.

Francois Fillon, right, a centre-right French presidential election candidate, campaigns in in Lille. Photo / AP
Francois Fillon, right, a centre-right French presidential election candidate, campaigns in in Lille. Photo / AP

According to Francetvinfo, authorities advised Fillon to wear a bullet-proof vest in Nice, which he refused. He is being guarded by 12 officers and has a direct line to the prime minister for security updates, it said.

But Fillon was not the only candidate informed of a threat. The security teams of Marine Le Pen, the far-right Front National candidate, and Emmanuel Macron, the centrist frontrunner, were shown police photos of the two suspects last Friday. Benoit Hamon, the Socialist candidate was also informed.

Le Pen is due to hold what may be her last campaign rally in Marseille tomorrow. "Perhaps it was no coincidence," said a member of her team.

With France going to the polls to choose a new president on April 23 and May 7, more than 50,000 police, gendarmes and soldiers are being deployed for each round, in particular around the country's 67,000 polling stations.

The country has been under a state of emergency, which has been extended several times, in the face of Islamist militant attacks in Paris and other parts of the country in which more than 230 people have been killed.

Five terror plots have been foiled since the start of the year and 17 were thwarted in 2016, Bernard Cazeneuve, the French Prime Minister confirmed last month.

Despite the threat, corruption, rather than terrorism, has dominated much of the presidential campaign until now.

After the arrests, Fillon said: "Democracy must not bend before the threats and intimidation of terrorists", adding that he hoped security would not now dominate the electoral debate.

The French were more concerned about unemployment and general insecurity rather than a "miscellaneous news item", he insisted.

But Le Pen, who has tacked hard-right this week, calling for France to suspend all legal immigration and draft in army reservists to guard French borders, seized on the arrests as proof France required a more authoritarian leader.

"The French must realise what's going on. For the past two five-year (presidential) terms, the Islamist threat has been allowed to prosper" leading to a "devastating multiplication of attacks and threats of attacks", she claimed.

"It's time to put back France in order."

The presidential race is narrowing ahead of the vote, with the pack closing behind Macron and Le Pen.

They are tied on 22-23 per cent, with Fillon improving to around 21 per cent and Jean-Luc Melenchon, the surprise Communist-backed contender, surging as high as 20 per cent in some polls.

The far-left firebrand staged his last major rally with a flourish, holding a meeting in Dijon, Burgundy, along with six others around France in which his hologramme appeared live. Tens of thousands were due to turn out.

Fillon, for his part, received a boost when ex-president Nicolas Sarkozy, who has avoided the campaign trail, issued a strong call to back him on Sunday to "turn the disastrous page of the last five years".

Macron, who hopes to win over left and right-wingers, aped one of Sarkozy's trademark calls for the French to "work more to earn more" during a visit to Rungis, Europe's biggest wholesale food market.

He pledged to simplify the country's famously complex labour laws within weeks of taking office.