The French presidential race frontrunner, Emmanuel Macron, has filed a lawsuit over claims that he has a secret bank account in the Caribbean.
The news came after he was regarded as having come out on top in the final TV debate ahead of Sunday’s run-off vote.
His far-right adversary, Marine Le Pen, referred to the claim in the debate.
“We will not hesitate to prosecute for defamation anyone who repeats this false information,” an aide to Mr Macron told AFP news agency.
During Wednesday night’s televised debate, Ms Le Pen referred to rumours that have been circulating online.
“I hope that we will not find out that you have an offshore account in the Bahamas,” she said to Mr Macron, the centrist, pro-EU candidate.
He replied: “That is slander.”
Judicial sources quoted by AFP said prosecutors in Paris had opened an investigation following a complaint from Mr Macron.
Speaking on French radio on Thursday, Mr Macron said the “fake news and lies” were from “sites, some of which were linked to Russian interests”.
His supporters say some of the sites have supported US President Donald Trump.
What happened in the debate?
Mr Macron and Ms Le Pen traded insults for more than two hours, arguing over terrorism, the economy and Europe.
Ms Le Pen lambasted him on his record as economy minister in the Socialist government – a post he quit to form his En Marche! movement.
The National Front candidate accused him of being “the candidate of savage globalisation” and said his version of France “is a trading room, where it will be everyone fighting for themselves”.
In turn, Mr Macron said Ms Le Pen had openly lied, proposed nothing and exaggerated the concerns of the public.
Ms Le Pen accused him of complacency about the threat of radical Islamist terrorism. “Security and terrorism are major issues that are completely missing from your programme,” she said.
In response, Mr Macron said the measures she proposed – “eradicating” Islamic fundamentalism by shutting down extremist mosques, and expelling preachers of hate – were “snake oil” that played into terrorists’ hands and the desire they had for a “civil war”.
They also clashed on the future of the European Union – another issue where they are clearly opposed.
Ms Le Pen has said she would call for an in-out referendum on EU membership, and in recent days declared the euro currency finished.
In an interview after Wednesday evening’s debate, Ms Le Pen said she had wanted to show French voters that Mr Macron was part of the old system.
Mr Macron said the debate had exposed the true nature of Ms Le Pen and her party.
Macron the clear favourite: Hugh Schofield, BBC Paris correspondent
This turned into a tense, at times incendiary exchange, between two leaders with starkly opposing ideas about the road France needs to take.
Marine Le Pen, who lags badly in the opinion polls, had clearly decided that the best tactic was to launch a full-frontal attack on Emmanuel Macron.
Constantly throughout the debate she threw insults and allegations at him, accusing him of being part of the discredited existing order.
But there was little substance to her attacks, and over and again Mr Macron was able to expose the weaknesses in her arguments – especially over the economy and the euro.
- Read more from Hugh: ‘Unworthy’ debate was still great viewing
Both candidates were hoping to make an impression on the estimated 18% of undecided voters in the first election the country has ever held without a candidate from the two traditional mainstream parties.
BFMTV found viewers who had watched the debate had a more favourable view of Mr Macron than Ms Le Pen in most categories.
He was the “most convincing” of the pair in the opinion of 63% of those interviewed.
Mr Macron was also deemed the “most convincing” for two-thirds of those who voted for both left-wing candidate Jean-Luc Mélenchon in the first round, and for 58% of those who voted for Republican François Fillon.
Mr Macron also led among voters when they were asked about which candidate was most honest, was most aligned with the voters’ values and had the best plans.
The BFMTV poll was carried out among 1,314 people over the age of 18 who had watched the television debate.
How has the French press reacted?
French newspapers were taken aback by the open hostility on display during Wednesday night’s debate.
Le Figaro saw Ms Le Pen adopt a “strategy of total war that consisted of bombarding her opponent without respite”. Mr Macron, it says, “dominated his opponent on economic issues” during a debate of “unprecedented brutality”.
Liberation accuses Ms Le Pen of “drowning the debate in an avalanche of disinformation” and publishes a list of what it says are false or exaggerated statements made by her.
Le Monde also firmly comes down on the side of Mr Macron, who it says “repeatedly denounced the ‘nonsense’ by the candidate of the National Front – often rightly”.
Les Echos takes issue with the aggressive tone of the debate, saying the candidates “spent more time attacking each other” than explaining future reforms.
Last updated April 25, 2017
*Polling results up to this date show how people said they would vote on 7 May, if Macron and Le Pen reached the second round
The polling average line looks at the five most recent national polls and takes the median value, ie, the value between the two figures that are higher and two figures that are lower.
French election: Macron takes action over offshore claims