Le Pen would close borders and quit the euro currency, while Macron wants closer European cooperation and an open economy.
Voters in some French overseas territories and the Americas were due to cast their ballots on Saturday, a day before voting in France itself. The first polling stations to open at 1000 GMT were in Saint Pierre and Miquelon, islands off Canada.
Others in French Guiana in South America; Guadeloupe and Martinique in the Caribbean; the South Pacific islands of French Polynesia and French citizens living elsewhere in the Americas were also due to vote on Saturday.
In France, police union Alternative Police warned in a statement that there was a risk of violence on election day by activists of the far-right or far-left.