The contest will be a rematch of the 2017 fight that Macron, a former investment banker who ran a reformist campaign from the centre, won handily at 66 percent compared with 34 percent for Le Pen.
But the result will be much closer this year, surveys suggest, and much will hinge on campaign arguments over the next two weeks highlighted by a prime-time television debate between the two candidates on April 20.
An Ifop-Fiducial poll conducted after polls closed on Sunday showed Macron with just a slight edge of 51 percent in the run-off compared to Le Pen's 49 percent, with a margin of error of three percentage points.
Separate polls by Ipsos-Sopra Steria as well as OpinionWay both put Macron at 54 percent against 46 percent for Le Pen.
Macron came out on top in the first round on Sunday, with a projected score of 28.6 to 29.7 percent, followed by Le Pen on 23.5 to 24.7 percent, using projections by polling firms based on samples from voting stations.
Final results will be published by the Interior Ministry in the coming hours.