French authorities took a hard line Saturday on what presidential front runner Emmanuel Macron called a "massive" hacking attack on his campaign, warning on the eve of the vote that anyone spreading the information could be committing a crime.
The warning came after the pro-Europe centrist's team lambasted a "massive and coordinated" hack that resulted in the online leak of thousands of emails, accounting details and internal documents late Friday.
It was an unexpected 11th-hour twist ahead of Sunday's decisive run-off after a bruising and divisive campaign pitting the 39-year-old former banker who embraces free-trade against his anti-EU, far-right rival Marine Le Pen.
"The dissemination of such data, which have been fraudulently obtained and in all likelihood may have been mingled with false information, is liable to be classified as a criminal offence," France's electoral commission said in a statement.
The documents spread on social media just before midnight as the candidates officially wrapped up campaigning, in what Macron's team termed an attempt at "democratic destabilisation, like that seen during the last presidential campaign in the United States".
Hillary Clinton has alleged Russian hacking of her campaign's emails was partly to blame for her defeat by Donald Trump in the US presidential election in November.
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Macron's team said the files were stolen weeks ago when several officials from his En Marche party had their personal and work emails hacked -- in one of "an intense and repeated" series of cyber-attacks targeting Macron since the launch of the campaign.
"Clearly, the documents arising from the hacking are all lawful and show the normal functioning of a presidential campaign," aides said in a statement.
But they warned that whoever was behind the leak had mixed fake documents with real ones "in order to sow doubt and disinformation".
The WikiLeaks website on Friday posted a link to the documents on Twitter, saying it had not yet discovered fakes in the cache of files and "we are very skeptical that the Macron campaign is faster than us."
Last month cybersecurity research group Trend Micro said Russian hackers called Pawn Storm had targeted Macron's campaign, using "phishing" techniques to try and steal personal data.
Senior Le Pen aide Florian Philippot suggested on Twitter that the leak might contain information the media had deliberately suppressed.
The election watchdog advised media not to publish details from the documents, warning that publication could lead to criminal charges and that some of the documents were probably fake.
The upset came at the end of a frantic final day of campaigning and as fresh security concerns emerged following the arrest of a suspected extremist.
Polls released earlier Friday had showed Macron gaining momentum, forecasting victory for the pro-European, pro-business former banker with around 62 percent to 38 percent for Le Pen.
He and Le Pen -- who is hoping to ride a global wave of anti-establishment anger to the Elysee Palace -- have offered starkly different visions for France during a campaign that has been closely watched in Europe and around the world.
Three days before the first-round vote in April, a policeman was shot dead in an Islamic State-claimed attack on Paris's Champs-Elysees, and jitters rose again on Friday when police arrested a suspected extremist near a military airbase.
Guns were discovered as well as a pledge of allegiance to IS and several of the group's flags, sources close to the case told AFP after the arrest in Evreux, north of Paris.
The suspect, a 34-year-old Muslim convert, was arrested early Friday after police found his car near the base.
In another major security breach on Friday, Greenpeace activists partially scaled the Eiffel Tower to hang a giant anti-Le Pen banner saying "Liberty, Equality, Fraternity" and "#resist".
Paris police said the stunt exposed "flaws" in the security surrounding the world-famous monument, and city authorities announced immediate measures to reinforce patrols at the site.
Le Pen has tried to portray Macron as being soft on security and Islamic fundamentalism, playing to the concerns of many of her supporters after a string of terror attacks in France that killed more than 230 people since 2015.
She has said she wants to copy Britain's example and hold a referendum on France's EU membership, sending alarm bells ringing in capitals across the bloc.
In the initial round of voting, she finished second with 21.3 percent after softening the FN's image over the past six years -- but without fully removing doubts about the party's core beliefs.
A former economy minister under Socialist President Francois Hollande, Macron quit the government last August to concentrate on his new political movement En Marche, which has drawn 250,000 members in 12 months.