PARIS — On the eve of the most consequential French presidential election in decades, the staff of the centrist candidate Emmanuel Macron said late Friday that the campaign had been targeted by a “massive and coordinated” hacking operation, one with the potential to destabilize the nation’s democracy before voters go to the polls on Sunday.
The digital attack, which involved a dump of campaign documents including emails and accounting records, emerged hours before a legal prohibition on campaign communications went into effect. While the leak may be of little consequence, the timing makes it extremely difficult for Mr. Macron to mitigate any damaging fallout before the runoff election, in which he faces the far-right candidate Marine Le Pen, who has pledged to pull France out of the euro and hold a referendum to leave the European Union.
The hacking immediately evoked comparisons to last year’s presidential election in the United States, during which American intelligence agencies have concluded that Russia’s president, Vladimir V. Putin, ordered an “influence campaign” to benefit the Republican nominee, Donald J. Trump. Groups linked to Russia have been accused of trying to hack the Macron organization.
Groups linked to Russia, that are also believed to have been involved in the hacking related to the 2016 United States presidential campaign, have previously been accused of trying to breach the Macron organization. Security experts tracking the activity of suspected Russian hackers say they believe those same groups were involved in this latest attack.
In a statement, the Macron campaign said the hackers had mixed fake documents along with authentic ones, “to sow doubt and misinformation.”
“Intervening in the final hour of the official campaign, this operation is clearly a matter of democratic destabilization, as was seen in the United States during the last presidential campaign,” the statement said.
Earlier on Friday, Ms. Le Pen’s campaign staff said its website also had faced “regular and targeted” attacks during the campaign.
It also said that the French authorities had investigated the attacks and, this week, arrested a suspect who was “close to extreme-left circles” and who had admitted to being responsible for several attacks on Ms. Le Pen’s campaign website. The arrest occurred on French soil, the statement said.
Unlike Mr. Macron, however, it did not appear that Ms. Le Pen’s campaign documents had been compromised. The authorities did not immediately confirm the Le Pen camp’s account.
The Macron campaign said the documents leaked Friday were stolen several weeks ago after the personal and professional emails of staff members at En Marche, his political movement, were hacked.
It was not the first reported hacking attempt of Mr. Macron’s campaign. In April, a report by the cybersecurity firm Trend Micro said there was evidence that the campaign was targeted in March by what appeared to be the same Russian operatives who were responsible for hacks of Democratic campaign officials before last year’s American presidential election. Mr. Macron’s campaign said the attack was unsuccessful.
Trend Micro said the attack involved sending emails to campaign officials and others with links to fake websites designed to bait them into turning over passwords.
In the attack reported on Friday, Vitali Kremez, the director of research at Flashpoint, a business risk intelligence company in New York, said he suspected the involvement of a Russian-linked espionage operation known as APT28. “The key goals and objectives of the campaign appear to be to undermine Macron’s presidential candidacy and cast doubt on the democratic electoral process in general,” he said.
“If indeed driven by Moscow, this leak appears to be a significant escalation over the previous Russian operations aimed at the U.S. presidential election, expanding the approach and scope of effort from simple espionage efforts towards more direct attempts to sway the outcome,” Mr. Kremez added.
Security researchers who have been tracking APT28, also known as Fancy Bear, say it has been moving aggressively against NATO members and a variety of Western targets using various hacking tools, including so-called spear-phishing attacks, but also through the exploitation of vulnerabilities in technologies that allow hackers to invade their targets undetected by security software.
The security researchers, who asked for anonymity to discuss an investigation, say they believe APT28 has previously passed stolen information taken from breached accounts to another Russian group. It is that second group of Russian cybercriminals that, in other instances, has published leaked information, security experts say, in large part to distance Russian intelligence operatives from the hacks and maintain a measure of plausible deniability.
There was no firm evidence that APT28 was involved in the theft of the Macron campaign documents.
Candidates and their staff are prohibited by French electoral rules from campaigning between midnight on Friday and when the last polls close at 8 p.m. Sunday — meaning that they cannot give interviews to the news media, hold rallies, make speeches or issue statements.
The Macron campaign made its statement about the hacking at 11:56 p.m. — four minutes before the legal prohibition on campaigning went into effect.
Numerama, a French online publication focusing on digital life, said that the hacked material appeared to have been disseminated through users of 4Chan, an online bulletin board. They shared nine gigabytes worth of documents and emails — “a jumble of what appears to be the contents of a hard drive and several emails of co-workers and En Marche political officials.”
“It will take to time to sift through it all, but at first glance, they seem to be utterly mundane,” Numerama said after analyzing the data. “One finds memos, bills, loans for amounts that are hardly over-the-top, recommendations and other reservations, amidst, of course, exchanges that are strictly personal and private — personal notes on the rain and sunshine, a confirmation email for the publishing of a book, reservation of a table for friends, etc.”
Some of the documents did not appear to have any link to Mr. Macron, Numerama reported.
The National Commission for Control of the Electoral Campaign, a regulatory body, said it was contacted by the Macron campaign on Friday night. The commission, which planned to meet on Saturday about the hacking, urged the news media to be cautious in its reporting.
“It therefore asks the media, and in particular their websites, not to report on the content of these data, recalling that the dissemination of false information is liable to fall within the scope of the law — in particular criminal law,” the commission said.
The Macron campaign appealed to journalists to not do the hackers’ bidding by widely publicizing the contents of the emails.
“We call upon the news outlets that wish to report on this operation to shoulder their responsibilities, in all good conscience,” the campaign said. “Indeed, this is not a simple hacking operation but well and truly an attempt to destabilize the French presidential election. It is therefore important to consider the nature of the leaked documents, to be fully aware of the fact that a large part of them are purely and simply fake, and the appropriateness of giving an echo to this destabilization operation.”
Gérard Araud, the French ambassador to Washington, said of the attack, in a post on Twitter that was later deleted: “It was to be expected. The last-ditch offensive benefiting a candidacy favored by a foreign government.”
One of Ms. Le Pen’s top allies, Florian Philippot, the vice president of her party, the National Front, asked on Twitter: “Will the #Macronleaks learn things that investigative journalism has deliberately killed,” referring to his argument that the mainstream news media has been biased against Ms. Le Pen. “Scary, this democratic shipwreck.”
Mr. Macron, 39, an independent centrist and a former economy minister who is making his first run for elected office, has been favored in the polls over Ms. Le Pen, 48, a lawyer and political heir to the National Front movement her father, now estranged, founded in 1972. The party has taken strong stands against immigration, globalization and the European Union.
The two candidates confronted each other Wednesday night in a vicious debate that more resembled an American-style shoutfest than the Descartian discourse that French voters are more accustomed to. Ms. Le Pen, who was relentless in her attacks, was widely judged the loser.
A post-debate poll, by the firm Ipsos — which was very accurate in its projections before the first round of the elections on April 23 — found Mr. Macron’s lead over Ms. Le Pen had widened, 63 percent to 37 percent.
But Ms. Le Pen could benefit on Sunday if poor and disaffected suburban voters do not cast ballots or if on-the-fence voters who are unenthusiastic about their choices decide to abstain.