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

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

“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.

The data leak emerged as polls predicted Mr. Macron was on course for a comfortable victory over far-right leader Marine Le Pen in Sunday's election, with the last surveys showing his lead widening to around 62 % to 38.

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

However, its 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.

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

Destabilisation

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

It was not immediately clear who was responsible, 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.”

France is the latest nation to see a major election overshadowed by allegations of manipulation through cyber hacking after U.S. intelligence agencies said in January 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.

Sunday's election is seen as the most important in France for decades, with two diametrically opposed views of Europe and the country's place in the world at stake.

The National Front's Ms. Le Pen would close borders and quit the euro currency, while independent Mr. Macron wants closer European cooperation and an open economy.

On Friday night as the #Macronleaks hashtag buzzed around social media, Florian Philippot, deputy leader of the National Front, tweeted “Will Macronleaks teach us something that investigative journalism has deliberately kept silent?"

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.

APT28 last month registered decoy internet addresses to mimic the name of En Marche, which it likely used send tainted emails to hack into the campaigns computers, Mr. Kremez said. Those domains include onedrive-en-marche.fr and mail-en-marche.fr.

“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 said.

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