Macron, who wants to deregulate the economy and deepen EU integration, is set to win the head-to-head with between 61.5 and 63 percent of the vote, according to the last opinion polls on Friday.
Should an upset occur and Le Pen win, the very future of the EU could be on the line given her desire to close borders, dump the euro currency, and tear up trade treaties.
Even in defeat, the 48 year-old's vote is likely to be about twice what her party scored the last time it reached the presidential second round in 2002, demonstrating the scale of voter disaffection with mainstream politics in France.
By midday, both candidates had voted, he in Le Touquet on the north coast, and she in the northern town of Henin-Beaumont.
Midday turnout figures from the Interior Ministry said 28.23 percent of voters had turned out so far, the lowest at this stage of the day since the 2002 presidential poll, when it was 26.19 percent. Turnouts at midday in 2012 and 2007 were 30.66 percent and 34.11 percent respectively.
A poll on Friday had predicted a final turnout of 75 percent this time. The eventual turnouts in 2002, 2007 and 2012 were all above 80 percent.
Pollsters see likely abstentions as highest among left-wing voters who feel disenfranchised by Sunday's choice after nine other candidates were eliminated in first round, but it is unclear what a high or low turnout could mean for the outcome.
Nevertheless, voter surveys forecasting the result itself proved accurate for the tight first round race last month.
Markets have risen in response to Macron's widening lead over his rival after a bitter television debate on Wednesday.
"We increased our equity exposure and added some French stocks after the first round," said Francois Savary, chief investment officer at Geneva-based fund management firm Prime Partners. "The major political risk of a Le Pen victory appears to be disappearing."
After a campaign in which favourites dropped out of the race one after the other, Le Pen is nevertheless closer to elected power than the far right has been in France since World War Two.
If opinion polls prove accurate and the country elects its youngest-ever president rather than its first female leader, Macron himself has said himself he expects no honeymoon period.
Close to 60 percent of those who plan to vote for Macron say they will do so to stop Le Pen from being elected to lead the euro zone's second-largest economy, rather than because they fully support the former banker turned politician.
"I don't necessarily agree with either of the candidates," psychotherapist Denise Dulliand, who was voting in Annecy in the mountainous southeast, told Reuters.
"But I wanted to express my voice, to be able to say that I came, even if I am really not satisfied with what is happening in our country, and that I would like to see less stupidity, less money and more fraternity."