Moments before midnight on Friday, the cut-off time for the official end of the campaign, Mr Macron's En Marche! issued a press statement announcing it had been "the victim of a massive, co-ordinated act of piracy".

However, the media blackout enforced in advance of Sunday's election will limit the effect of the leak on the result.

Tens of thousands of emails, accounts, photos and contracts obtained through the hack were distributed over social media and on a site calling itself "Macronleaks" on Friday evening, France time.

WikiLeaks helped the pirates by publishing a link to copies of the documents, in case the original release was taken down.

They appear to have been hacked several weeks ago from the email accounts of several people close to the En Marche! team, including left-wing radical Alain Tourret and Macron speechwriter Quentin Lafay.

However, En Marche! claimed that "false documents" had been mixed into genuine ones "in order to sow doubt and misinformation".

WikiLeaks doubted this claim, saying on Twitter it had "not yet discovered fakes in #MacronLeaks and we are very sceptical that the Macron campaign is faster than us". It speculated that Russia might be behind the leak because it "needs hostility".

Florian Philippot, an aide for Mr Macron's opponent Marine Le Pen, said on Twitter that it was a "frightening democratic shipwreck".

However, he also promoted the hashtag #Macronleaks, and speculated the leak may "uncover things that investigative journalism has deliberately killed". The Le Pen campaign has repeatedly complained the French media were biased against them.

Previous hacking attacks on En Marche! were traced to Russia, the campaign has said, though it did not say where the latest attack came from.

The En Marche! statement said "This operation is obviously aimed at destabilising democracy, as has already been seen in the United States during their last presidential campaign."

"Throughout this campaign En Marche! has consistently been the movement most targeted by such [hacking] attempts, intensively and repeatedly."

En Marche! asked media to be careful in reporting the documents, as "a large part of them are purely and simply false", and others related to provisional accounts and estimates rather than campaign commitments.

Mr Macron goes into Sunday's vote a clear favourite, with France poised to reject his opponent's far right, anti-immigrant, anti-Islam populism.

Unless polls are stunningly wrong, the centrist newcomer has survived a few missteps and a surge of fake news attacks and will comfortably beat the Front National's Le Pen.

Not only that, the 39-year-old is on track to push his barely year-old party, En Marche!, to become the largest in the Parliament after elections later this year, giving him the power to choose a prime minister who will assist rather than oppose his agenda.

Immediately after his win two weeks ago in the first round of voting, he stumbled: he looked too celebratory at a fancy bistro dinner with supporters after the result, then he was ambushed by Ms Le Pen at a public appearance at a factory where they duelled over their diametrically opposed policies for France's manufacturing future, with Ms Le Pen positioning herself as the protector of the workers.

Meanwhile, Ms Le Pen, 48, stepped down as leader of the far-right Front National and made some moves towards the political centre.

But the latter days of the campaign saw her looking more strident and desperate.

That tone was exposed in a fiery, 160-minute televised debate on Wednesday evening, one of the most brutal political exchanges in recent French history.

Ms Le Pen has remained a stubborn 20 points behind in the polls, and would need to persuade almost every undecided voter to back her - or Mr Macron's current supporters to decide to stay home instead.

So she took a go-for-broke attitude into the debate, goading her opponent, seeking to unsettle him. She flung insults at him, calling him "the darling of the system and the elites", the "candidate of savage globalisation" and the "representative of subjugated France".

"We've seen the cynical choices you've made, that reveal the coldness of the investment banker you have never ceased being," she said. "You defend private interests ??? and behind that there is social ruin."

This aggressive language has appealed to her base of supporters during the earlier weeks of the campaign, but it was judged by many post-debate commentators as off-putting to the political middle ground that Ms Le Pen would have to win to gain the Elysee Palace.

And polling confirmed the pundits, with about two-thirds of the audience marking Mr Macron as more credible in the debate.

Mr Macron had returned fire at Ms Le Pen, calling her the "high priestess of fear" and accusing her of lying to the public. French media repeated this line with approval the next day: Le Monde's front page headline was "The strategy of the lie", comparing Ms Le Pen's rhetoric to that of US President Donald Trump.

"What's extraordinary is that your strategy is simply to say a lot of lies and propose nothing to help the country," Mr Macron told Ms Le Pen. "Don't say stupid things. You are saying a lot of them."

Ms Le Pen was most comfortable, and Mr Macron least, on the topic of terrorism and security. Ms Le Pen accused her rival of complacency, and Mr Macron replied with a rambling discourse on the social origins of terrorism. He also accused Ms Le Pen of provoking "a civil war" with her proposals to ban religious symbols including Muslim headscarves from public places.

But Ms Le Pen stumbled on her own proposal to bring back the French franc.

And at the end of the debate Ms Le Pen suggested Macron had "an offshore account in the Bahamas", a claim she later admitted she could not prove.

Macron told her it was "defamation" and pointed out that Ms Le Pen herself was under investigation for alleged fraudulent payments from the European Parliament to the Front National's staff.

He later said the Bahamas rumour had been planted and promoted by the Kremlin's social media troll farms, to favour the Putin-friendly Ms Le Pen.

"This is typical of fake news," Mr Macron told a radio interviewer. "Marine Le Pen ??? has troops behind her on the internet who take up positions. Some of these sites are linked to Russian interests . . . People talked and got organised."

It is not the first time a rash of social media rumours about secret bank accounts have been boosted by Russian internet pirates and automated Twitter accounts, Mr Macron's team said. They also claimed there had been 4000 hacking attempts against their team's computers, most originating in Russia.

Alt-right activists from the US, who rallied behind Donald Trump's campaign last year, have also tried to influence debate and trends online. However a study by the New York Times found that its language and iconography was not hitting the target with the French.

Groups on the internet's notorious '4Chan' forum tried to push the Bahamas rumour on the day of the debate. One anonymous user posted screen shots of documents supposed to prove the existence of Mr Macron's Bahamas company.

The user claimed they had "sent these to hundreds of French journalists and they've all sat on this ??? anybody even talking about this in France has been shut down ??? Let's get grinding. If we can get #MacronCacheCash trending in France for the debates tonight, it might discourage French voters from voting Macron".

On Friday prosecutors in Paris opened an investigation following a complaint from Mr Macron, and his campaign warned they would "prosecute for defamation" anyone who repeated "this false information".

Mr Macron's campaign was forced to issue a denial, saying the documents - and the news - were clumsy fakes. But the denial meant the claims were aired in genuine news outlets.

Mr Macron also last week received the backing of former US president Barack Obama, who provided a video message to his campaign in which he said he said the French election was "important to the future of France and the values we care so much about".

"I've admired the campaign that Emmanuel has run, he stood up for liberal values," Mr Obama said. "He appeals to people's hopes and not their fears."

The final days of the campaign saw polls suggesting Mr Macron had extended his lead against Ms Le Pen.

On Friday Ms Le Pen was jeered by protesters during a visit to the famous Reims cathedral, forced to flee through a side door to avoid the crowd.

Meanwhile, a confident Mr Macron discussed who he would choose as his prime minister, looking ahead to critical parliamentary elections in June.

A poll on Thursday found that En Marche! was on track to win 249-286 seats, just shy of the 290 required for a majority.

It would be an extraordinary result for a party that was only brought into being in the last year.

And it would be another blow for the country's former leading parties, the Socialists and the Republicans, currently left on the sidelines of the country's biggest political fight.