France's far-right National Front has accused authorities of staging a media stunt to influence the presidential election after police searched its headquarters in an investigation into "fake jobs".

The searches came after French government bond yields rose sharply on news of a poll showing leader Marine Le Pen gaining ground on her main election rivals, independent Emmanuel Macron and conservative former prime minister Francois Fillon.

On Friday Ms Le Pen denied allegations by OLAF, the European Union anti-fraud agency, that she gave parliamentary assistants fake jobs paid for out of EU funds.

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 had Ms Le Pen easily beating her four main rivals to win the 23 April first round with 27% of the vote.

In the second-round two-way run off against Mr Macron or Mr Fillon, she was still seen losing, but both scenarios saw her narrowing the gap.

She would lose against Mr Macron with 42% to his 58%, while against Mr Fillon she would be defeated with 44% to his 56%, the poll showed.

A week ago she was polling around 36-37% against Mr Macron.

It is not clear what impact the probe could have on Ms Le Pen or how quickly the investigation will move forward.

Unclear who Le Pen may face in run-off

Mr 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.

With nine weeks to go, it is unclear which candidate would go through to the knockout against Ms Le Pen.

Both Mr Macron and Mr Fillon are polling at around similar levels, according to several surveys.

Things may become clearer on Wednesday when veteran centrist Francois Bayrou will announce whether he will enter the race.

Mr Bayrou, a pro-EU politician who won 18.5% of first-round votes in the 2007 presidential vote, is polling around 5%.

He has accused Mr Fillon of being under the influence of "financial powers" and said French democracy is under threat.

Mr Macron, whose aides call for a union with Mr Bayrou, is a political novice who has never held elected office.

However, he has pulled in huge crowds at rallies, saying he seeks to transcend the classic left-right divide in French politics.

Polls see little chance of a Socialist revival in time for the election given Socialist President Francois Hollande's poor record.

Markets have been uneasy about a possible agreement between hard-left Socialist candidate Benoit Hamon, who is polling at around 15%, and independent far-left candidate Jean-Luc Melenchon, at around 12%, which could strengthen Ms Le Pen in the second round.

But moves late last week to form an election deal between the two men appeared to have fizzled out.

Mr Melenchon, who is standing as an independent, said: "I have no intention of going and hitching myself to a hearse."

Mr Hamon hit back at the weekend, telling journalists: "I won't run after Jean-Luc Melenchon. I don't run after anyone."