Voting is under way in the final round of France’s presidential race after a massive online dump of frontrunner Emmanuel Macron’s campaign data delivered a final dramatic twist to the country’s most bruising, divisive and significant election in decades.

The French election watchdog warned that it could be a criminal offence to publish the tens of thousands of hacked emails and other documents – some reportedly fake – amid an electioneering blackout lasting from midnight on Friday until polls close at 8pm on Sunday.

The interior ministry said turnout at 5pm was 65.3%, noticeably down on 71.96% at the last election in 2012 but in line with pollsters’ projections ahead of the vote. The final abstention rate is set to be 25-27%.

The hack, on which neither Macron or his opponent, far-right leader Marine Le Pen, were allowed to comment publicly, was “clearly an attempt at democratic destabilisation, like that seen during the last presidential campaign in the US”, according to his En Marche! campaign team.

The divisive election to choose the Fifth Republic’s eighth president has turned the country’s politics upside down, with neither of the two mainstream centre-right and centre-left movements that have governed France since the second world war making it to the runoff.

Seen as potentially the most important electoral contest in many years for France and the European Union, it has pitted against each other two candidates with diametrically opposing visions for the future of their country and the continent.

Macron, a 39-year-old former banker and economy minister running as an independent centrist, is economically liberal, socially progressive, globally minded and upbeat. Le Pen is a nation-first protectionist who wants to close France’s borders and possibly leave the euro and the EU.

Macron voted in the coastal town of Le Touquet in northern France alongside his wife, Brigitte, smiling and stopping to pet a black dog as he stepped out of his holiday home in the seaside resort.

For security reasons, he was driven to his nearby polling station at Le Touquet town and shook hands with a large crowd of supporters before entering the building. Le Pen her ballot in Hénin-Beaumont, a small northern town run by her Front National.

She arrived at the polling station with the town’s mayor, Steeve Briois, who took over as the far-right party’s interim leader when she stepped aside last month to concentrate on the campaign.

Feminist activists were briefly detained a couple of hours earlier on Sunday for hanging a big anti-Le Pen banner from a church in the town.

Marine Le Pen emerges from a pooling booth. Photograph: Pascal Rossignol/Reuters

Final polls published on Friday suggested Macron had widened his lead over Le Pen to between 22 and 23 percentage points following an ugly TV debate in which she was widely considered to have spent more time attacking her opponent than engaging with policy.

“The commission calls on everyone present on internet sites and social networks – primarily the media, but also all citizens – to show responsibility and not pass on this content so as not to distort the sincerity of the ballot,” the election commission said on Saturday.

Many television news channels opted not even to mention the hack, while Le Monde newspaper said on its website it would not publish any of the nine gigabytes of leaked data before the election – partly because there was too much, and partly because it had clearly been released with the 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,” the paper said.

The data was posted on a profile called EMLEAKS to Pastebin, a site allowing anonymous document sharing. En Marche! said it was not alarmed by the content, adding the documents “reflected the normal operations of a campaign” but had been mixed with fakes to “sow doubt and disinformation”.

Intelligence agencies in the US said in January that the Russian president, Vladimir Putin, had ordered hacking of Democratic campaign officials before last year’s presidential election in a bid to boost the chances of Republican candidate Donald Trump over Hillary Clinton.

Mainland France’s 47 million voters began casting their ballots at about 70,000 polling stations around the country at 8am (0700 BST). Usually reliable estimates of the result, based on a representative count of actual votes cast, will be released as the last stations close at 8pm (1900 BST).

Voting began in France’s overseas territories on Saturday, starting with Saint-Pierre-et-Miquelon, an archipelago near the Canadian island of Newfoundland, and continuing in other far-flung overseas territories and French embassies abroad.

Up to a quarter of the electorate is expected to abstain, with some supporters of the centre-right candidate François Fillon and the hard-left veteran Jean-Luc Mélenchon, both defeated in the first round on 23 April, saying they would not be voting for either candidate.