"The files circulating were obtained several weeks ago due to the hacking of the personal and professional mailboxes of several party officials," Macron's En Marche! (On The Move) party said in a statement, just as campaigning officially ended ahead of Sunday's election.
Macron's team added that all the documents were "lawful" and said the large-scale hacking was "unprecedented in a French electoral campaign".
The online leak shortly before midnight on Friday came barely 24 hours before Sunday's final round of the presidential election - one of the country's most dramatic in decades - in which the centrist former economy minister faces far-right leader Marine Le Pen.
"The En Marche movement was victim of a massive and coordinated hacking attack leading to the spreading this evening on social media of internal information of a diverse nature (emails, accoutning documents, contracts)," the statement said.
"Those circulating these documents are adding many false documents to authentic documents in order to sow doubt and disinformation," Macron's team said.
WATCH: Greenpeace unveils anti-Le Pen banner on Eiffel Tower
It noted that the release came "in the last hour of the official campaign" and that it is "clearly a matter of democratic destabilisation, like that seen during the last presidential campaign in the United States".
"Throughout the campaign, En Marche! has constantly been the party the most targeted by such attempts, in an intense and repeated fashion," the lengthy statement said.
"The aim of those behind this leak is, all evidence suggests, to hurt the En Marche! party several hours before the second round of the French presidential election."
It went on: "Clearly, the documents arising from the hacking are all lawful and show the normal functioning of a presidential campaign.
The WikiLeaks website posted a link on Twitter to the trove of documents -- saying it "contains many tens of thousands (of) emails, photos, attachments up to April 24, 2017" -- while indicating it was not responsible for the leak itself.
WikiLeaks said there were around 9 gigabytes of data in total, saying it was "checking parts" of the cache.
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Pro-European Macron and anti-immigration Le Pen offered starkly different visions for France during a campaign that has been closely watched in Europe and the rest of the world.
Anti-racism protesters hurled objects at Le Pen and ally Nicolas Dupont-Aignan as they visited a cathedral in northern Reims, shouting "Marine, give back the money!"
She and her National Front (FN) party are being targeted by several investigations, including one into misuse of expenses in the European Parliament.
Reims is deeply intertwined with French history and Joan of Arc -- a nationalist symbol adopted by the FN.
"Mr Macron's supporters behave violently, even at Reims cathedral, a symbolic and sacred place. No dignity," Le Pen wrote on Twitter.
Macron, who faced boos and heckles by leftist trade unionists as he visited a factory in the southern Tarn region on Thursday, went to a cathedral in southern Rodez on his last day of campaigning.
WATCH: Who is Emmanuel Macron?
Arrest near airbase
Security was brought to the fore again with news that police had arrested a suspected extremist close to an military airbase northwest of Paris.
Guns were discovered as well as a pledge of allegiance to the Islamic State (IS) and several of the group's flags, sources close to the case told AFP on condition of anonymity after the arrest in Evreux.
The suspect, a 34-year-old Muslim convert, was arrested early Friday after police found his car near the base.
An extremist shot and killed a police officer and wounded two others on Paris's Champs Elysees avenue three days before the first round of the presidential election on April 23 in an attack claimed by IS.
In a major security breach, Greenpeace activists partially scaled the Eiffel Tower on Friday to hang a giant anti-Le Pen banner saying "Liberty, Equality, Fraternity" and "#resist".
The Paris police department said the early-morning protest had exposed "flaws" in the security surrounding the world-renowned monument.
City and police officials announced immediate measures to reinforce patrols at the site, and plans to re-evaluate a video surveillance system.
Le Pen has tried to portray Macron as being soft on security and Islamic fundamentalism, playing to the concerns of many of her supporters after a string of terror attacks in France that have killed more than 230 people since 2015.
Le Pen sees her rise as the consequence of growing right-wing nationalism and a backlash against globalisation reflected in the election of US President Donald Trump and Britain's shock vote to leave the European Union.
She has said she wants to copy Britain's example and hold a referendum on France's EU membership, sending alarm bells ringing in capitals across the bloc.
In the first round of the election, she finished second behind Macron on 21.3 percent after softening the FN's image over the past six years -- but without fully removing doubt about the party's core beliefs.
Macron is a pro-EU, pro-business and pro-trade reformer who quit the government last August to concentrate on his new centrist political movement En Marche, which has drawn 250,000 members in 12 months.
He won high-profile backing from former US president Barack Obama on Thursday, who said in a video that Macron "appeals to people's hopes and not their fears".
Speaking Friday, Macron said he had already chosen his future prime minister -- but even the person concerned had not been informed.
"Yes, this choice has been made 'in petto'," he told Europe 1 radio, using an Italian expression meaning "in my heart".
Macron said he would only announce his choice after he took over from President Francois Hollande, if he wins.
Le Pen has said she would appoint Dupont-Aignan -- a eurosceptic who was knocked out in the election's first round -- as her premier if she becomes president.