French presidential candidate Emmanuel Macron's campaign says it has been the target of a "massive" computer hack that dumped its emails online, just over 24 hours before voters go to the polls to choose between the centrist and his far-right rival Marine Le Pen.

Macron's political movement En Marche! (Onwards!) said the release of thousands of emails, accounting documents and other files was an attempt at "democratic destabilisation, like that seen during the last presidential campaign in the United States".

Some nine gigabytes of data were posted by a user called EMLEAKS to Pastebin, a document-sharing site that allows anonymous posting, on Friday.

"The En Marche! Movement has been the victim of a massive and coordinated hack this evening which has given rise to the diffusion on social media of various internal information," the movement said in a statement.

Kenneth Grey, a retired FBI special agent and lecturer at the University of New Haven, told Al Jazeera he is not surprised by the hacking attack.

"It certainly does seem to be the new way to try to affect politics in other countries," he said. "Hack into their email, release the contents, and if there is embarrassing information there, it may very well cause a swing in the election."

He added that "unless there is a smoking gun within these emails", he doesn't believe it will have an effect on Sunday's election result.

Campaign blackout

News about the email dump began to appear on Friday evening just hours before the official ban on campaigning went into effect at midnight.

The campaign blackout means that candidates and media now face restrictions until polls close on Sunday evening.

On Saturday, the electoral commission warned media organisations not to republish any information hacked from Macron's campaign team, saying it could be a criminal offence.

Macron's team said the files were stolen weeks ago when several officials from En Marche! had their personal and work emails hacked - one of "an intense and repeated" series of cyberattacks against Macron since the launch of the campaign.

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The WikiLeaks website posted a link on Twitter to the trove of documents, saying it was not responsible for the leak but that it was "examining" parts of the cache.

The hashtag #MacronLeaks swiftly began trending worldwide on Twitter.

Last month a cybersecurity research group said Russian hackers called Pawn Storm had targeted Macron's campaign.

Japan-based Trend Micro said the group had used "phishing" techniques to try to steal personal data from Macron and members of his campaign.

Ahead in polls

Opinion polls published on Friday - in advance of the campaign blackout - suggested independent centrist Macron was 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 latest surveys show him winning with about 62 percent of the vote.

Former economy minister Macron's team has previously blamed 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 Macron's camp renewed complaints against Russian media and a hackers' group operating in Ukraine.

In its statement on Friday, En Marche! said that the documents released online showed only the normal functionings 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.

University of New Haven's Grey told Al Jazeera that "if it is an attack by the Russians against Macron, it's really an attack against the EU".

"It may be bigger than France," he said.

US intelligence agencies said in January that Russian President Vladimir Putin had ordered hacking of the Democratic National Committee and the chairman of Hillary Clinton's Democratic campaign to influence the election on behalf of Donald Trump, her Republican rival who went on to win the US presidency.