French police were searching the headquarters of Marine Le Pen's National Front party west of Paris on Monday in relation to an alleged "fake jobs" probe.
The European parliament's anti-fraud agency, Olaf, has said that, in her role as French National Front leader Le Pen had during the 2011-12 legislature paid party staff with EU funds, which EU rules say should be used only to pay EU lawmakers' assistants.
But the FN accused the authorities of staging a media stunt to discredit them.
"It looks on the face of it like a media operation whose goal is to disturb the course of the presidential campaign," the National Front said in a statement.
French judges opened a fraud investigation on 15 December after prosecutors handed the dossier over to them following a preliminary investigation of more than a year.
"This is as void as space," the party's vice-president Florian Philippot told BFM television, adding that searches had taken place a year ago and nothing had been found then.
"These are media-stunt searches on the day when she (Le Pen) gets a 2-point bounce in the polls. It's always when the system is in panic that these affairs come out."
An Opinionway poll of voting intentions on Monday had Le Pen easily beating her four main rivals to win the April 23 first round with 27 percent of the vote.
In the second-round two-way runoff against Macron or Fillon, she was still seen losing, but both scenarios saw her narrowing the gap.
She would lose against Macron with 42 percent to his 58, while against Fillon she would be defeated with 44 percent to his 56, the poll showed. A week ago she was polling around 36-37 against Macron.
It is not clear what impact the probe could have on Le Pen or how quickly the investigation will move forward.
Fillon's status as favourite to win the presidency in May has evaporated in the past three weeks amid questions about what work his wife did for hundreds of thousands of euros in taxpayers' money when she was paid as his assistant.
He has vowed to fight on despite falling ratings and the threat of being placed under formal investigation by the financial police, who are handling the matter.
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