The largest march was in eastern Paris, centered around the Place de la Bastille, site of the revolutionary-era prison that is a symbol of French democracy. At least 400,000 people of all ages and classes of society chanted anti-Le Pen slogans, held up banners, played instruments or beat drums to reggae beats.
Didier Hughes, 56, an economist, called Le Pen "a fascist, and so dangerous for France that we all must unite." He added: "I've not seen this kind of atmosphere on the streets for 30 years."
Elsewhere in France, more than 900,000 others marched in a dozen cities, including Grenoble, Lyon, Bordeaux, Marseille, Toulouse and Strasbourg.
That's the biggest turnout in what have been daily protests since the extreme-rightwing leader won a place in the decisive second round of presidential elections, reports CBS News Correspondent Elaine Cobbe in Paris.
In the Paris protest, good-natured crowds shouting "Down with Le Pen!" packed the streets near the site of the former Bastille prison. One demonstrator, 20-year-old Abdoul Fofana, said she was worried about Le Pen's fierce anti-immigrant stance.
"If Le Pen wins there will be a world war in France," said Fofana, who came to France from the Ivory Coast 10 years ago.
Le Pen, who has incorporated Joan of Arc's campaign against England into his anti-immigrant stance, held his own, much smaller rally earlier in the day. His supporters packed the Rue de Rivoli, a main artery through the capital, chanting "Le Pen, president" and waving tricolor flags and signs that said "I'm proud to be French." Then they marched to the Paris Opera in an annual May Day event that took on added importance this year because of Le Pen's surprise candidacy.
Le Pen lay a bouquet of white flowers at a gilded statue of Joan of Arc riding a horse and waving the national flag. For Le Pen's National Front party, the 15th century peasant girl who led a series of victories against the English is a symbol of French resistance against foreign "invaders."
In a speech, Le Pen promised an "electoral earthquake" in the election's final round, which pits him against conservative incumbent President Jacques Chirac, who is expected to win easily.
"The ground's going to crumble under their feet," he said.
Police and observers estimated the pro-Le Pen crowd at 10,000 to 12,000 people, though Le Pen's party claimed there were as many as 100,000 marchers.
Wednesday was expected to be the climax of growing protests against Le Pen, and marches were planned in some 70 cities around France.
Before the massive anti-Le Pen protest got underway in Paris, similar demonstrations were drawing tens of thousands. More than 50,000 people gathered in the eastern city of Grenoble, while 45,000 demonstrated in Bordeaux, police said. Some of the protests were combined with traditional May Day labor protests by unions.
Since Le Pen's surprise success in the April 21 first round, the nationalist leader has complained he has been a "victim of a campaign of hate and lies." His daughter, also a National Front politician, said the march was the "proper response" to the wave of anti-Le Pen street protests that have swept France.
"The problem is not the demonstrations, it's the defamation, the slander and the insults that people shout," said Marine Le Pen. "It's shameful and scandalous."
Le Pen has been convicted of racism and anti-Semitism numerous times. He blames immigration, particularly from Muslim North Africa, for unemployment — which edged up in March to 9.1 percent — and for rising crime. His success in the April 21 first round of elections stunned France and most of its allies and neighbors.
The far-right leader wants to pull France out of the European Union and return to the franc, the currency abandoned in favor of the euro at the start of this year. He also supports deporting all illegal immigrants and tightening border controls.