The campaign of French presidential candidate Emmanuel Macron says it has been the target of a "massive hacking attack" after a trove of documents was released online.
The campaign said that genuine files were mixed up with fake ones in order to confuse people.
It said that it was clear the hackers wanted to undermine Mr Macron ahead of Sunday's second round vote.
The centrist will face off against far-right candidate Marine Le Pen, with results expected from 6am Monday NZT.
The documents were leaked on a file sharing website late on Friday, as the official presidential campaigning period drew to a close.
What was released?
About nine gigabytes of data were posted online by an anonymous user.
The details are unclear so far, but Mr Macron's En Marche movement said internal campaign documents, including emails and financial data, had been taken in an "act of massive, co-ordinated hacking".
"The leaked files were obtained several weeks ago by hacking personal and professional email accounts of several officials of the movement," it said in a statement.
The campaign said the documents showed only legitimate campaign activities.
France's election authorities have warned media outlets against reporting on the contents of the leaked documents because of the restrictions now in place.
Who might be responsible?
That too remains unclear. The Macron camp has not blamed any specific party but said the hack clearly aimed to damage it and undermine French democracy.
Last month security experts from the company Trend Micro said that Russian hackers were targeting Mr Macron's campaign.
Russia has denied that it is behind attacks aimed at Mr Macron.
On Thursday, the centrist candidate filed a lawsuit over online rumours that he had a secret bank account in the Caribbean.
Mr Macron called the allegations "fake news and lies" and said some of the sites spreading them were "linked to Russian interests".
What else happened on the final day of campaigning?
Separate security alerts in and around Paris marred Friday's final scramble by the candidates to court voters.
A suspected radical Islamist possessing weapons and a pledge of allegiance to IS was arrested north of the capital.
And Greenpeace activists scaled the Eiffel Tower to unfurl a banner, sparking an emergency police meeting.
What is at stake on Sunday?
France's voters have rejected the two big political parties - the Socialists and the Republicans - that have governed for decades,
Voters will be making a decision on France's future direction and on its place at the heart of the European Union.
If they opt for liberal Emmanuel Macron, they will be backing a candidate who seeks EU reform as well as deeper European integration, in the form of a eurozone budget and eurozone finance ministers.
If instead they choose far-right Marine Le Pen she promises quite the opposite. She wants a Europe of nations to replace the EU.
"I give myself six months to negotiate with the EU the return of sovereignty. Then it will be the French who decide," she tweeted.
The assumption is that she would fail and a referendum would take place initially on France's membership of the euro.
After the Brexit vote in the UK and the election of US President Donald Trump, France is the latest country to deal a blow to politics as usual.
What are the battleground issues?
One of the overriding issues facing French voters is unemployment, which stands at almost 10% and is the eighth highest among the 28 EU member states. One in four under-25s is unemployed.
The French economy has made a slow recovery from the 2008 financial crisis and all the leading candidates say deep changes are needed.
Marine Le Pen wants the pension age cut to 60 and to "renationalise French debt", which she argues is largely held by foreigners.
Emmanuel Macron wants to cut 120,000 public-sector jobs, reduce public spending by €60bn, plough billions into investment and reduce unemployment to below 7 percent.
What about security?
The election is taking place amid a state of emergency, and the first round took place three days after a policeman was shot dead on the Champs Elysées in the heart of Paris.
More than 230 people have died in terror attacks since January 2015 and officials fear more of the hundreds of young French Muslims who travelled to Syria and Iraq may return to commit new atrocities.
Intelligence services believe attackers are deliberately pursuing a Le Pen victory, says the BBC's Hugh Schofield in Paris - because that could tip the country into chaos.
The former FN leader wants to suspend the EU's open-border agreement on France's frontiers and expel foreigners who are on the watch lists of intelligence services.
What happens after Sunday?
Mr Macron and Ms Le Pen came top of the 11 candidates in total who participated in the first round of voting on 23 April.
While the outcome of Sunday's second round should be clear that evening, the results will be officially proclaimed by France's constitutional council on Thursday, 11 May.
Sunday, 14 May, marks the end of outgoing President François Hollande's term, and is the latest possible date for the inauguration and official transfer of power to his successor.