Emmanuel Macron's centrist campaign has said it was the victim of a "massive and co-ordinated" hacking attack, as France prepares to choose its next president on Sunday.
Emmanuel Macron, who founded his En Marche! movement only last year, will duel with the far-right's Marine Le Pen for the keys to the Elysee Palace.
In a statement late on Friday, En Marche! said it was hacked a few weeks ago, and that the leaked documents had been mixed with false ones to "seed doubt and disinformation" and destabilise Sunday's vote.
The online pirate attack led to the leak of campaign emails and financial documents, Mr Macron's team said.
Responding to the news, WikiLeaks tweeted that the leak contained "many tens of thousands" of emails, photos and attachments dated up to April 24.
But it said that it had come "too late" to affect the election results.
The candidates stopped campaigning at midnight on Friday to give voters a day of reflection before the poll, which has already seen the two parties that have dominated post-war politics cast aside by voters.
Polls suggest Mr Macron, 39, will win comfortably, with the defeated Gaullist and Socialist candidates endorsing him to succeed Francois Hollande with appeals for national unity against Ms Le Pen's Front National.
Mr Macron and Ms Le Pen won through to the run-off after a first-round last month which saw nine other candidates eliminated.
Socialist candidate Benoit Hamon managed a meagre 6.4% of the vote in his doomed bid to keep the Elysee in the hands of the centre-left party, coming fifth.
Francois Fillon, of the centre-right Les Republicains, was pipped to second place by Ms Le Pen having entered the race as favourite for the Gaullist movement, which has held power for 33 of the 59 years since France adopted its Fifth Republic constitution, giving sweeping powers to the president.
His campaign was torpedoed by corruption claims surrounding payments to his wife Penelope, a Welshwoman, for work prosecutors claim was never done.
Mr Fillon resisted calls to step aside when the scandal emerged and his party paid the price with elimination at the first round.
Mr Macron and Ms Le Pen fought out an ill-tempered debate on Wednesday on live TV but the last polls of the campaign suggest the far-right candidate will lose by around 60% to 40%.
Both candidates visited cathedrals on the last day before the weekend, when campaigning does not take place.
Pro-European Mr Macron went to Rodez cathedral while nationalist Ms Le Pen was in Reims, but was jeered as she left via a back door.
Mr Hollande won the Elysee back for the Socialists in 2012 after 17 years of Gaullist presidents but decided not to seek a second term after five years plagued by terror attacks that killed hundreds and forced him to impose a state of emergency.
Mr Macron, his finance minister until last year, broke with the Socialists and set up his own movement which he insists is neither left nor right.
The fact neither a Gaullist or Socialist is in the run-off is unprecedented since France adopted its Fifth Republic system in 1958.
Traditionally centre-left wins in presidential polls are celebrated at the Place de la Bastille and centre right triumphs across Paris at the Place de la Concorde.
Mr Macron's team, in a bid to show it is not aligned to either side, reportedly sought permission to use the Champs de Mars behind the Eiffel Tower on Sunday for what he hopes will be a victory rally but permission was refused in order to protect the lawns that are part of Paris's Olympic bid.