The Paris prosecutor's office says it has opened a preliminary investigation into whether false news is being used to influence voting in Sunday's French presidential election run-off.

Prosecutors started the probe after centrist presidential candidate Emmanuel Macron filed a lawsuit on Thursday, making a legal complaint over rumours circulating on social media that he had hidden wealth in offshore accounts.

Macron's camp said the former investment banker was victim of a "cyber misinformation campaign".

Macron sued after far-right rival Marine Le Pen suggested in a televised debate on Wednesday that he could be holding an offshore account in the Bahamas. Le Pen backed away from the suggestion on Thursday.

A spokesman for Macron's En Marche! (Forward) party said the complaint to Paris prosecutors was not against Le Pen, even though "it happens that she mentioned it", but against persons unknown, according to the DPA news agency.

Le Pen has repeatedly sought to portray Macron as the candidate of high finance and "the system", pointing to his four years working at the Rothschild investment bank.

Al Jazeera's Natacha Butler, reporting from Albi, in southern France, said that Macron had vehemently denied the allegations of having an offshore account.

"He says it's not true and has filed this legal claim in order to say that it's not correct in this presidential campaign to fling these sorts of [false] allegations around because they can be so damaging," said Butler.

"Because, of course, if he was to have such a bank account, that would be something that most voters would be quite annoyed about - it's a time in France where unemployment is high, where so many people do struggle to make ends meet."

Rough treatment on campaign trail

Macron and Le Pen both got rough receptions on Thursday as they returned to the campaign trail after their bad-tempered debate the previous evening.

Protesters threw eggs at Le Pen as she visited a town in the western region of Brittany, C-News television reported.

Footage showed the National Front leader's security detail crowding around her as they rushed her into a factory she was due to visit.

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Macron was meanwhile taken to task by a crowd of trade union members when he visited a glassworks in Albi, on the second-last day of campaigning ahead of Sunday's vote.

Amid a chorus of boos, Macron was interrogated about his controversial plans to liberalise labour laws.

Le Pen's team says hacker attacked website

Also on Thursday, Le Pen's team released a statement claiming that a hacker had confessed to repeatedly targeting the campaign's website.

The statement gave few details about the seriousness of the interference, which could range from attempts at defacing the website to flooding it with bogus traffic. It also did not give the precise timing of the arrest, saying only it happened "this week".

Police referred questions to prosecutors, who did not immediately return a message seeking comment. The Le Pen campaign did not immediately respond to a request for comment, reported the Associated Press news agency.

There has been intense anxiety in France over the possibility that hackers could tamper with France's high-stakes run-off.

Opinion polls give Macron an 18-to-22-percentage-point lead over Le Pen ahead of Sunday's poll.