France’s electoral commission warned media and internet users that they could face criminal prosecution for publishing documents obtained in a “massive and coordinated hacking attack” on the presidential frontrunner Emmanuel Macron’s political movement.

The commission, which held an emergency meeting to discuss the leak, said some of the documents appeared to contain “false information”.

Thousands of internal En Marche! (On the Move!) documents were published on the internet an hour before the Friday midnight deadline for the start of a campaign blackout, meaning neither Macron nor his far-right Front National rival Marine Le Pen was allowed to respond.

France’s presidential electoral authority, the CNCCEP, asked the media to avoid transmitting information from the leaked documents and reminded them of their responsibilities given the “seriousness of the election”.

“This attack has resulted in the publication of a number of important documents presented as having come from the information system of the candidate and the message accounts of certain of their campaign officials on certain social networks,” it wrote after the meeting on Saturday. “The commission stresses that the dissemination or republication of such information, fraudulently obtained and which may, in all likelihood, have been mixed with false information, is liable to be classified as criminal in several respects for which its authors will be held responsible.”

It added: “On the eve of the most important electoral deadline for our institutions, we call on all actors present on websites and social networks, first and foremost the media, but also all citizens, to show a spirit of responsibility and not relay the contents of these documents in order not to alter the integrity of the vote, not to break the bans laid down by the law and not to expose themselves to the committing of criminal offences.”

Le Monde said it had seen part of the documents. It said the hacking attack was “clearly aimed at disturbing the current electoral process”. The paper said it would not publish the content of any pirated document before the second round vote was over and the results known at 8pm on Sunday.

About 9GB of data was posted by a user called EMLEAKS to the document-sharing site Pastebin, which allows anonymous posting. It was not immediately clear who was responsible.

The documents were posted as #MacronLeaks on social networks in the .eml format and linked to Pastebin. Le Monde reported that the first documents were relayed via the 4chan forum, which it said was favoured by far-right American groups and on English-language, pro-Trump Twitter accounts. They were then relayed to WikiLeaks.

En Marche! confirmed the hack, saying it had been the “victim of a massive and coordinated hack … which has given rise to the diffusion on social media of various internal information”.

Five opinion polls published on Friday forecast that Macron would win the election with a share of 62-63%, comfortably defeating his Front National rival Marine Le Pen in France’s most turbulent and potentially significant presidential race in decades.

A leading Front National official, Florian Philippot, asked on Twitter whether the leaked documents “would reveal anything that investigative journalism had kept quiet”.

Macron’s team has previously blamed Russian interests for repeated attempts to hack its systems during the campaign, saying on 26 April it had been the target of unsuccessful attempts to steal email credentials since January. The Kremlin has denied any involvement.

During a visit by Le Pen to Moscow in March, Vladimir Putin said Russia had no intention of meddling in the elections, but analysts said the meeting implicitly signalled support for the Front National.

En Marche! banned certain Russian media outlets from campaign events.

Trend Micro, a cybersecurity firm, said last month that a hacking group, believed to be part of a Russian intelligence unit, was targeting Macron and his campaign team, adding that it appeared to be the same Fancy Bear group behind the hacking of Democratic campaign officials before last year’s US presidential election.