Two men were arrested in Marseille on Tuesday on suspicions of preparing an "imminent, certain" attack in French "in the coming days", Interior Minister Matthias Fekl said.
Police held the two, aged 23 and 29, "on suspicion of planning violent act on the eve of presidential election", Xinhua news agency quoted Fekl as saying.
They were known to intelligence services for radicalisation, he added.
An ongoing search operation in Marseille has so far uncovered materials on the planned attack, according to the minister.
Reiterating the government's "relentless fight against terrorism", Fekl pledged that "every effort is made to ensure security of the election".
A total of 50,000 policemen and gendarmes will be mobilised during the two rounds of presidential elections on April 23 and May 7.
France remains in a state of emergency and on high alert over possible terrorist attacks.