French Presidential favourite Emmanuel Macron's campaign team has claimed it has been the victim of "massive and co-ordinated hack".
A large trove of emails were among around nine gigabytes of data shared by a user called EMLEAKS to Pastebin, a document-sharing site that allows anonymous posting.
Researchers from a Japanese anti-virus firm claim the centrist politician has been targeted by Russian hackers.
The leaked data was posted just a day and a half days before voters go to the polls to decide who will be France's next leader.
Mr Macron faces his far-right rival Jean Marine Le Pen in the presidential run-off on Sunday.
The politician formed his own party, En Marche! to fight the election on a centrist platform and is the bookies' favourite to beat Ms Le Pen.
A statement from the party said: "The En Marche! Movement has been the victim of a massive and co-ordinated hack this evening which has given rise to the diffusion on social media of various internal information."
His team is understood to be claiming the 'leaks' are a mixture of genuine documents and fabricated ones.
Comments about the email dump began to appear just hours before the official ban on campaigning began. The ban is due to stay in place until the last polling stations close on Sunday at 8pm.
Wikileaks, an organisation which publishes leaks, said on Twitter that it was "looking into" whether the leaks are genuine.
The organisation added that the leaks are believed to contain many thousands of photographs, attachments and emails.
France's Interior Ministry would not comment on the reported hacking of hundreds of emails because official campaigning has ended.
"Neither the ministry, nor any other ministry would be commenting on this because according to the law, campaigning has ended as of midnight," a spokesman said.
During a stormy television debate this week, Mr Macron accused Ms Le Pen of using "fake news" to try and manipulate the outcome of the election after she alluded to reports, believed to be fake, that he has an offshore bank account.
Mr Macron has launched legal action over allegations that he holds an offshore account, although the Paris’ prosecutor’s office said no one was named in Mr Macron’s complaint, which has triggered an inquiry into the suspected spread of false stories aimed at influencing the election.Reuse content