"We have come here to protest against the farce of this election," a demonstrator told AFP.
Several hundred young people rallied in Bastille Square — the historic site where the French Revolution began in 1789 — after projections suggested that the runoff election would be between Le Pen and Emmanuel Macron, a former investment banker and economy minister of socialist President Francois Hollande.
A protest leader called on the public through a loudspeaker to rally "against Marine and Macron." Three people were arrested, according to police.
Another anti-fascist demonstration took place late Sunday in the western coastal city of Nantes.
Up to 60 percent of active police officers planned to vote for the xenophobic and anti-immigration candidate Le Pen, according to a Cevipof poll released in January.
In recent years, Le Pen has increasingly been attracting French youth, with a quarter of 18-34-year-olds stating they "would like to see her win the election" in a survey carried out in early March by Harris Interactive. The same proportion wished to see neoliberal banker Emmanuel Macron as president, while 19 percent supported leftist Jean-Luc Melenchon.
Le Pen's father, Jean-Marie, scored the first breakthrough in the French presidential elections for the National Front, winning a slot in the runoff in 2002.
He lost in a head-to-head contest to the center-right incumbent, Jacques Chirac.