The French media and public have been warned not to spread details about a hacking attack on presidential candidate Emmanuel Macron, reports the BBC.
Strict election rules are now in place and breaching them could bring criminal charges, the election commission said.
A trove of documents - said to mix genuine files with fake ones - was released online shortly before campaigning ended on Friday.
The centrist Mr Macron faces far-right candidate Marine Le Pen on Sunday.
It is part of the restrictions that came into force at midnight local time on Friday.
No campaigning or media coverage of it that could sway the election is allowed until polls close at 20:00 local time (18:00 GMT) on Sunday. Some overseas French territories have already begun voting.
The election commission warned it could be a criminal offence to republish the leaked data.
Politicians and media are forbidden from giving details of, or commenting on, the leak.
The election commission said in a statement on Saturday: "On the eve of the most important election for our institutions, the commission calls on everyone present on internet sites and social networks, primarily the media, but also all citizens, to show responsibility and not to pass on this content, so as not to distort the sincerity of the ballot."
Analysts say that, given the open flow of social media content, policing this could be impossible.
The French daily Liberation covered the leak by publishing a general Q&A with a journalist.
Le Monde said it would "not publish the contents before the second round". It said it would vet and publish relevant material but "respecting our journalistic and ethical rules". It also carried a general Q&A of the leaks.
Florian Philippot, deputy leader of Ms Le Pen's National Front party, got a tweet in before the rules came in, saying: "Will Macronleaks teach us something that investigative journalism has deliberately kept silent?"