Le Pen is the daughter and political heir to Jean-Marie Le Pen, the founder of the National Front, a far-right, nationalist, anti-immigration party.
She has led in the opinion polls so far, but pollsters predict she will find it much harder to defeat an opponent in the second round of voting, where mainstream parties often work to prevent the National Front from winning.
Mélenchon, a onetime Trotskyite and former Socialist politician, left the Socialist Party in 2008 to create the Left Party, backed by the Communist Party.
Hamon, a lawmaker and former education minister, left the government when his party shifted its position toward pro-business, austerity policies.
Although he is a member of the Left Party, he is running as the candidate for La France Insoumise, an umbrella political movement he created.