France has sought to keep a computer hack of frontrunner Emmanuel Macron's campaign emails from influencing the outcome of the country's presidential election, warning that it could be a criminal offence to republish the data.

Key points: As much as 9 gigabytes of data was posted online

As much as 9 gigabytes of data was posted online The commission, which supervises the electoral process, says data was fraudulently obtained

The commission, which supervises the electoral process, says data was fraudulently obtained Intelligence expert says review indicates Russian military intelligence behind the leak

Mr Macron's team said a "massive" hack had dumped emails, documents and campaign financing information online just before campaigning ended on Friday (local time) and France entered a quiet period, which forbids politicians from commenting on the leak.

French electoral laws impose a news blackout on Saturday and for most of Sunday on any campaigning and media coverage seen as swaying the election.

"On the eve of the most important election for our institutions, the commission calls on everyone present on internet sites and social networks — primarily the media, but also all citizens — to show responsibility and not to pass on this content, so as not to distort the sincerity of the ballot," the French election commission said in a statement.

Hack probably won't impact vote We know the fact they've called in the cyber security agency means that it must have been massive and very significant. They're only brought in on cases if they are big. And that's what the Macron campaign had suggested. Francois Hollande has spoken out and said he will deliver a response. They're going to take some kind of action. But the odd thing is because of this campaign restriction — and it occurred an hour before midnight on Friday — no-one has been able to talk about it. If you turned on the French television today, as I did, it wasn't mentioned. So not a lot of details. Because it hasn't had a lot of publicity in the mainstream media, the suggestion is it probably won't have an impact on the vote. But how much of it is being picked up online? How much of it is being pushed and spread by Mr Macron's rivals is a question that we won't know. But the reality is that the polls are so wide and appear to be so stable for him in the lead that it's going to take an extraordinary event for him not to take out the election. Analysis from Europe correspondent Lisa Millar

As much as 9 gigabytes of data was posted on a profile called EMLEAKS to Pastebin, a site that allows anonymous document sharing, late on Friday.

France's government cybersecurity agency will investigate the attack, according to a government official who said it appeared to be a "very serious" breach.

But outgoing French President Francois Hollande said the attack was not a surprise.

"We knew that this kind of risk would be present during the presidential campaign, because it has happened elsewhere. Nothing will be left without a response," he told French news agency AFP.

The data leak emerged as polls predicted Macron was on course for a comfortable victory over far-right leader Marine Le Pen in Sunday's election.

The last surveys showed his lead widening to around 62 per cent to 38.

Fake information mixed in leak to 'sow doubt'

The commission, which supervises the electoral process, said after a hastily called meeting on Saturday (local time) that the data had been fraudulently obtained and could be mixed with false information.

It was not immediately clear who was responsible for the hack, but Mr Macron's political movement said in a statement the hack was an attempt to destabilise democracy and to damage the party.

"The En Marche! (Onwards!) Movement has been the victim of a massive and co-ordinated hack," it said.

En Marche! said the leaked documents dealt with the normal operations of a campaign and included some information on campaign accounts.

It said the hackers had mixed false documents with authentic ones to "sow doubt and disinformation".

The Macron team asked the campaign oversight commission to bring in cybersecurity agency ANSSI to study the hack, according to a government official who spoke to The Associated Press on condition of anonymity.

ANSSI can only be called in for cases where the cyberattack is "massive and sophisticated" — and the Macron hack appears to fit the bill, the official said.

French media covered it in various ways, with left-leading Liberation giving it prominence on its website, but television news channels opting not to mention it.

The electoral commission's rules may be difficult to enforce in an era where people get much of their news online, information flows freely across borders and many users are anonymous.

Russia likely tied to leak: expert

France is the latest nation to see a major election overshadowed by allegations of manipulation through cyber hacking.

In January, US intelligence agencies said that Russian President Vladimir Putin had ordered hacking of parties tied to Democratic presidential candidate Hillary Clinton to influence the election on behalf of Republican Donald Trump.

The Kremlin has denied it was behind any such attacks, although Mr Macron's camp renewed complaints against Russian media and a hackers' group operating in Ukraine.

Vitali Kremez, director of research with New York-based cyber intelligence firm Flashpoint, told Reuters his review indicated that APT 28 — a group tied to the GRU, the Russian military intelligence directorate — was behind the leak.

APT 28 last month registered decoy internet addresses to mimic the name of En Marche!, which it likely used to send tainted emails to hack into the campaign computers, Mr Kremez said.

"If indeed driven by Moscow, this leak appears to be a significant escalation over the previous Russian operations aimed at the US presidential election," he said.

"… Expanding the approach and scope of effort from simple espionage efforts towards more direct attempts to sway the outcome."

Macron's campaign has previously complained about attempts to hack its emails, blaming Russian interests in part for the cyber attacks.

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