A large trove of emails from the campaign of French presidential candidate Emmanuel Macron was posted online late on Friday, a little more than a day before voters go to the polls to choose the country's next president in a run-off against far-right rival Marine Le Pen.
Some nine gigabytes of data were posted by a user called EMLEAKS to Pastebin, a document-sharing site that allows anonymous posting. It was not immediately clear who was responsible for posting the data or whether the emails were genuine.
In a statement, Macron's political movement En Marche! (Onwards!) confirmed that it had been hacked.
"The En Marche! Movement has been the victim of a massive and co-ordinated hack this evening which has given rise to the diffusion on social media of various internal information," the statement said.
In its statement on Friday, En Marche! said that the documents released online only showed the normal functioning of a presidential campaign, but that authentic documents had been mixed on social media with fake ones to sow "doubt and misinformation".
"The seriousness of this event is certain and we shall not tolerate that the vital interests of democracy be put at risk," it added.
Comments about the email dump began to appear on Friday evening just hours before the official ban on campaigning began. The ban is due to stay in place until the last polling stations close on Sunday at 8 p.m.
An interior ministry official declined to comment, citing French rules which forbid any commentary liable to influence an election, and which took effect at midnight French time on Friday.
Former economy minister Macron's team has already complained about attempts to hack it systems during a fraught campaign, blaming Russian interests in part for the cyber attacks.
On April 26, 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.
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.
Opinion polls show independent centrist Macron is set to beat National Front candidate Le Pen in Sunday's second round of voting in what is seen to be France's most important election in decades. The race features two candidates have diametrically opposed views of Europe and France's place in the world at stake.
The National Front's Le Pen would close borders and quit the euro currency, while independent Macron, who has never held elected office, wants closer European cooperation and an open economy.
The candidates of France's two mainstream parties, which have alternated in power for decades, were both eliminated in the first round of voting on April 23.