* Emails posted online just before official campaign ends
* Electoral commission warns republication may be a crime
* Polls predict Macron set for comfortable win over Le Pen
* Voting starts in overseas French territories (Adds Hollande comment, details on security)
By Adrian Croft and Geert De Clercq
PARIS, May 6 (Reuters) - France sought to keep a computer hack of frontrunner Emmanuel Macron's campaign emails from influencing the outcome of the presidential election, with the electoral commission warning on Saturday that it may 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, effectively forbidding politicians from commenting on the leak.
Polls have been predicting that Macron, a former investment banker and economy minister, is on course for a comfortable win over far-right leader Marine Le Pen in Sunday's election, with the last surveys showing his lead widening to around 62 percent to 38.
"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," French President Francois Hollande told French news agency AFP.
The election commission, which supervises the electoral process, warned social and traditional media not to publish the hacked emails lest they influence the vote outcome, but may find it difficult to enforce its rules in an era where people get much of their news online, information flows freely across borders and many users are anonymous.
"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 commission said in a statement on Saturday.
"The commission stresses that publication or republication of these data...could be a criminal offence," it said.
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.
Le Monde newspaper said on its website it would not publish the content of any of the leaked documents before the election, partly because the huge amount of data meant there was not enough time to report on it properly, but also because the dossiers had been published on purpose 48 hours before the election with the clear aim of affecting the vote.
"If these documents contain revelations, Le Monde will of course publish them after having investigated them, respecting our journalistic and ethical rules, and without allowing ourselves to be exploited by the publishing calendar of anonymous actors," it said. As the #Macronleaks hashtag buzzed around social media on Friday night, Florian Philippot, deputy leader of Le Pen's National Front party, tweeted "Will Macronleaks teach us something that investigative journalism has deliberately kept silent?"
As much as 9 gigabytes of data purporting to be documents from the Macron campaign were posted on a profile called EMLEAKS to Pastebin, a site that allows anonymous document sharing.
It was not immediately clear who was responsible, but Macron's political movement said in a statement the hack was an attempt to destabilise democracy and to damage the party.
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."
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.
Le Pen would close borders and quit the euro currency, while Macron wants closer European cooperation and an open economy.
Voters in some French overseas territories and the Americas were due to cast their ballots on Saturday, a day before voting in France itself. The first polling stations to open at 1000 GMT were in Saint Pierre and Miquelon, islands off Canada.
In France, police union Alternative Police warned in a statement that there was a risk of violence on election day by activists of the far-right or far-left.
Extreme-right student activists burst into the office of Macron's political movement in the southeastern city of Lyon on Friday evening, setting off smoke grenades and scattering false bank notes bearing Macron's picture, police said.
More than 50,000 police and some 7,000 soldiers will provide security during voting on Sunday, with more than 12,000 security personnel covering the Paris region alone.
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.
Vitali Kremez, director of research with New York-based cyber intelligence firm Flashpoint, said his review indicated that APT 28, a group tied to the GRU, the Russian military intelligence directorate, was behind the leak.
Macron's campaign has previously complained about attempts to hack its emails, blaming Russian interests in part for the cyber attacks.
The Kremlin has denied it was behind any such attacks, although Macron's camp renewed complaints against Russian media and a hackers' group operating in Ukraine.
(Additional reporting by Bate Felix, Andrew Callus, Myriam Rivet and Michel Rose in Paris, Catherine Lagrange in Lyon, Jim Finkle in Toronto and Eric Auchard in Frankfurt; Editing by Alexander Smith and Hugh Lawson)