French President Francois Hollande has promised to respond to the hacking of centrist presidential candidate Emmanuel Macron's campaign e-mails and the spreading of them online just one day before the May 7 runoff election.
Speaking in Paris on May 6, Hollande said: "We knew that there were these risks during the presidential campaign because it happened elsewhere. Nothing will go without a response."
Hollande said that if there has been "any interference or appropriations, there will be procedures which will begin," adding: "We need to let the investigations happen."
France's electoral commission has warned French journalists not to reveal the contents of the hacked e-mails on the last day before the election when no campaigning is authorized.
The documents were spread on social media on May 5 just before midnight in what Macron's team said was an attempt at "democratic destabilization, like that seen during the last presidential campaign in the United States."
Similar leaks of Democratic campaign documents on WikiLeaks and other online sites during the U.S. presidential election last year were eventually traced by U.S. intelligence agencies to hackers they said were working for the Russian government.
Hollande said he was unable to say whether it was an attempt at destabilizing the election, as alleged by Macron's team.
Macron faces far-right nationalist leader Marine Le Pen in the runoff.
The final public opinion polls, issued just hours before the hacked e-mails were released, suggested Macron would win the runoff with about 62 percent of the vote compared to 38 percent for Le Pen.
The electoral commission, which supervises the presidential campaign, said in a May 6 statement that the publication or republication of the information could be a criminal offense.
The commission's warning comes after Macron's campaign said on May 5 that it had been the target of a "massive" computer hack that dumped its campaign e-mails online 1 1/2 days before voters choose between the centrist and his far-right rival, Marine Le Pen.
"Free and fair elections are at play," the commission said in a statement, adding that some of the documents probably were fake and there could penalties -- even criminal ones -- for rebroadcasting forged documents. The commission issued the statement after an emergency meeting on May 6.
Macron's En Marche! (On The Move) party said in a statement that the leaks came "in the last hour of the official campaign" and were "clearly" aimed at "democratic destabilization, like that seen during the last presidential campaign in the United States."
Such a large-scale hacking is "unprecedented in a French electoral campaign," it said.
The WikiLeaks website posted a link on Twitter to the trove of documents, saying it "contains many tens of thousands [of] e-mails, photos, attachments up to April 24, 2017."
WATCH: High Stakes In French Presidential Runoff Vote
WikiLeaks indicated that it was not responsible for the leak itself but did not say where it obtained the documents.
As much as 9 gigabytes of data was posted on Pastebin, a document-sharing site that allows anonymous posting. The Pastebin document dump was posted by a user called EMLEAKS.
Front-runner Macron takes a hard line on maintaining EU sanctions imposed on Moscow over the Ukraine crisis, whereas Le Pen, who favors France leaving the EU, backs the lifting of sanctions and improving ties with Russia.
"The files circulating were obtained several weeks ago due to the hacking of the personal and professional mailboxes of several party officials," and then were released just as campaigning for the May 7 presidential runoff officially ended at midnight on May 5, Macron's campaign said.
While most of the leaked documents appear authentic, the campaign said, "those circulating these documents are adding many false documents to authentic documents in order to sow doubt and disinformation."
The campaign said the leaks are clearly aimed at boosting the election prospects of Macron's opponent, far-right nationalist leader Marine Le Pen.
"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 said.
"Throughout the campaign, En Marche! has constantly been the party the most targeted by such attempts, in an intense and repeated fashion," it said.
Despite the apparently massive effort to sway the election, none of the leaked documents contained anything potentially damaging or embarrassing to Macron, the campaign maintained.
"The documents arising from the hacking are all lawful and show the normal functioning of a presidential campaign," it said.
Whether there is time before the election for the public to even learn what's in the huge trove of documents was a question, however. WikiLeaks said there were around 9 gigabytes of data in total.
Moreover, French journalists are prohibited by law from publicizing the material in the hours left before the vote. Soon after the documents were released, France's electoral commission issued guidance asking French publications to refrain from covering them.
"Free and fair elections are at play," the commission said in a statement, adding that some of the documents probably were fake and there could be penalties -- even criminal ones -- for rebroadcasting forged documents.
Macron's campaign first disclosed it was under attack by hackers in February, and at that time it blamed a hackers' group operating out of Russia or Ukraine.
It did not say who could be behind the May 5 leaks. Russia has repeatedly denied any involvement or interference in the French election.
The French Interior Ministry declined to comment on the leaks, citing French rules which forbid any commentary that could influence an election in the day before the vote.
The last opinion polls taken before the election show Macron was heavily favored to win with about 62 percent of the vote.
The first voting stations opened on May 6 in the small territory of Saint-Pierre-et-Miquelon, off the eastern coast of Canada.
The territory is made up of two islands with a population of some 6,000 people.
French South American and Caribbean territories including Guiana and Martinique and French Polynesia also were voting on May 6.