Voters take to the polls tomorrow to decide who will be France's next president
French presidential candidate Emmanuel Macron's campaign team has claimed it has been the victim of a "massive hacking attack."
Hundreds of documents have been leaked online this morning.
Around 9GB of data was posted to a document-sharing site that allows anonymous posting - just hours before the end of official election campaigning.
The media has been warned not to publish the documents with France's electoral commission warning that any organisation that does so risks committing a criminal offence.
The Macron campaign said authentic internal documents have been mixed with fake ones on social media as part of attempts to sow "doubt and misinformation."
It said the original documents only show the "normal functioning" of a presidential campaign and accused those behind the attack of trying to destabilise Sunday's presidential run-off - comparing it to emails leaked from Hillary Clinton's US presidential campaign.
"Their publication makes internal documents public but has no reason to worry us as far as the legality and conformity of the documents is concerned," Mr Macron's campaign said in a statement.
"The seriousness of this event is certain and we shall not tolerate that the vital interests of democracy be put at risk."
An interior ministry official declined to comment, citing French rules which forbid any commentary liable to influence an election.
Mr Macron's team has already complained about attempts to hack its systems during its election campaign, blaming Russian interests in part for the cyber attacks.
On 26 April, the team said it had been the target of a series of attempts to steal email credentials since January, but that the perpetrators had so far failed to compromise any campaign data.
At the time, Mr Macron's head of digital campaigning Mounir Mahjoubi accused Russia of a combined attempt to hack the campaign's systems and spread fake news.
In February, the Kremlin denied that it was behind any such attacks, even though his camp renewed the charges against Russian media and a hackers' group operating in Ukraine.
Mr Macron, the centrist, and far-right rival Marine Le Pen stopped campaigning at midnight on Friday to give voters a day of reflection before they go to the polls on tomorrow.
8,000 French people in Ireland will vote at the French Embassy.