Polling day follows an unprecedented campaign marked by scandal, repeated surprises and a last-minute hacking attack on Macron, a 39-year-old who has never held elected office.
The run-off vote pits the pro-Europe, pro-business Macron against anti-immigration and anti-EU Le Pen, two radically different visions that underline a split in Western democracies.
Le Pen, 48, has portrayed the ballot as a contest between the "globalists" represented by her rival -- those in favour of open trade, immigration and shared sovereignty -- versus the "nationalists" who defend strong borders and national identities.
A first estimate of the results will be published around 1800 GMT.
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"The political choice the French people are going to make is clear," Le Pen said in her opening remarks during an often vicious debate between the pair on Wednesday night.
The last polling showed Macron -- winner of last month's election first round -- with a widening lead of around 62 per cent to 38 per cent before the hacking revelations on Friday evening. A campaigning blackout entered into force shortly after.
France's election authority said publishing the documents could be a criminal offence, a warning heeded by traditional media organisations but flouted by Macron's opponents and far-right activists online.
"We knew that there were these risks during the presidential campaign because it happened elsewhere. Nothing will go without a response," French President Francois Hollande told AFP on Saturday.
US intelligence agencies believe state-backed Russian operatives were behind a massive hacking attack on Democratic candidate Hillary Clinton's campaign ahead of America's presidential election last November.