French authorities are warning the media and citizens against sharing leaked details of a hack attack that targeted the country’s leading presidential candidate, Emmanuel Macron, ahead of Sunday’s election.

Just before a two-day media blackout and campaigning ban began in France, Macron’s representatives announced that a “massive and coordinated” hacking operation had occurred.

Thousands of documents, some real and some falsified, were reportedly shared online anonymously. Authorities have yet to find who is responsible, but the French election commission is investigating the hack attack.

In a statement on Saturday, the commission urged people not to disseminate any of the leaked information.

“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,” it read.

Macron, the independent centrist candidate, faces off against far-right rival Marine Le Pen in the vote on Sunday.

The election will be a critical test for both France and the European Union, as Le Pen has vowed to pursue an exit from the EU if she is elected president. Numerous polls in the lead-up to the vote showed Macron with a sizeable lead of around 24 percent.

The final week of the presidential race has seen a bitter contest unfolding. They engaged in heated cross-talk during a televised debate on Wednesday, often directly insulting each other and their policies. Le Pen has attempted to portray Macron as part of the globalist elite that has ignored the average French citizen, while he has accused her of lacking realistic policies and lying to voters.

Macron saw a slight bump in the polls following the debate, and is hoping to stave off a massive upset from Le Pen on election day. If he wins, he will become modern France’s youngest president and its first from a non-establishment party.

Le Pen is hoping to defy the polls and capitalize on a surge of support in recent years for her populist platform. France’s election race has seen a collapse of traditionally powerful parties, scandals and voters deeply divided over what path the country should take to address its many challenges.

France has grappled with major terrorist attacks and poor economic growth in recent years, as well as debate over issues such as immigration and integration. Sunday’s vote will be a decisive moment for the country and the EU, which is set to be closely watched around the world.