* Two French nationals arrested in southern city of Marseille
* Officials say leading candidates warned last week of risk
* France goes to polls on April 23 and May 7
Two Frenchmen arrested in Marseille on Tuesday planned an "imminent and violent attack" ahead of the first round of the presidential election on Sunday, France's interior minister said.
France's internal intelligence agency, which had been looking for the two suspects for more than a week, had warned main candidates in the election that there was a threat to their security, campaign officials said.
Comments by officials indicated that former prime minister Francois Fillon, centrist Emmanuel Macron and far-right leader Marine Le Pen - three leading contenders - were among those warned of a security risk.
"These two radicalised men ... intended to commit in the very short-term - by that I mean in the coming days - an attack on French soil," Interior Minister Matthias Fekl told a news conference.
A "definite terrorist act" had been foiled, he said.
France goes to the polls on April 23 and on May 7 in one of the most unpredictable elections in its modern history with security a key issue after attacks by militant Islamists which have killed more than 230 people in the past two years.
The two men - 23-year-old Clement Baur and 30-year-old Mahiedine Merabet - were seized in the southern port city a few moments apart from each other on Tuesday morning.
Local mayor Lisette Narducci said explosives had been found after a search of an apartment near Marseille's largest train station. Sources close to the investigation said at least two assault weapons had been found.
"The main candidates were warned in the middle of last week that there was a risk and that two individuals had been identified. We received their photos and security of the candidates. Their meetings and headquarters were given extra protection," Macron's spokesman Benjamin Griveaux told Reuters.
Officials from Fillon's camp and Le Pen's National Front said they also had received word of a possible attack.
With the country still under a state of emergency, Fekl said 50,000 police, gendarmes and soldiers would be deployed for each leg of the election, and security reinforced for the candidates.
"Everything has been put in place to ensure the security of this big event for our democracy and our Republic. The security forces are mobilised everywhere across France to ensure the security of French people and to ensure the presidential campaign goes smoothly," he said.
Le Pen, one of the frontrunners in the race for the Elysee, is scheduled to hold the last big rally of her campaign in Marseille on Wednesday, according to her programme.
Search warrants seen by Reuters described the two men as dangerous. "These are radicalised common law criminals who spent some time in prison," said one police source.