In the best ever performance for the anti-immigration, anti-European party, the National Front secured 30.6 percent of the vote nationally, an exit poll by Ifop-Fiducial poll showed, ahead of former President Nicolas Sarkozy's center-right Les Republicains party and their allies, who secured 27 percent.
As expected, President Francois Hollande's ruling Socialists came third polling 22.7 percent.
On Corsica an alliance of left-wing parties was in the lead. There will be a run-off on December 13.
If confirmed, the result would be a boost for Le Pen, who had hoped her anti-immigrant, anti-euro party would emerge on top in the first round, boosting her hopes for the 2017 presidential elections.
"The people of France have made it clear that they are fed up… with the uncontrolled influx of migrants and the terrorist threat. This is something the European leaders can’t deny, otherwise they will keep making the mistakes they made before and the threat of extremism will grow," the Flemish Nationalist Party's leader Bart De Wever said on Monday.
To resolve the EU’s migrant crisis De Wever, whose New Flemish Alliance formed Belgium’s ruling center-right coalition government in 2014, has proposed closing the Schengen borders and excluding Greece "if it keeps the doors open" to migrants from North Africa and the Middle East.
He has also called for an end to "the Turkish blackmail".
"If we do not do this the rift between various social groups in France and elsewhere in Europe will deepen even further and drive us to a dead-end," De Wever warned.