French authorities Saturday attempted to contain the fallout from a computer hack of Emmanuel Macron’s political party, less than a day before the country votes in a presidential election that pits him against far-right nationalist Marine Le Pen.

The commission overseeing the election met Saturday to discuss the hack, which became public when thousands of emails and internal documents from Mr. Macron’s party, En Marche, were uploaded anonymously to the internet Friday evening. The commission said the files were probably laced...