Le Pen said in the interview last week with The Associated Press that she would consider it a victory if she matched the first-round score of her father in 2002. That year, he got nearly 16.8 percent of the vote and was propelled into the final round and a face-off with then-President Jacques Chirac.
Anti-racism group SOS Racisme said Le Pen's victory could represent "a major danger for democracy." It blamed Sarkozy for allowing Le Pen's party to push extremist themes to the front of the national debate, and urged the candidates for the runoff "to put equality, more than ever, at the heart of their plans."
"This is an election that will weigh on the future of Europe. That's why many people are watching us," said Hollande after voting.
Whatever happens to France's leadership will affect the rest of the 27-nation European Union.
France was one of six countries that in the 1950s founded the predecessor of the EU, and is the eurozone's second-largest economy after Germany.
Sarkozy and German Chancellor Angela Merkel — a tandem that some call "Merkozy" — have championed a treaty on budget austerity for the 17-nation eurozone. But Hollande wants the treaty to also address economic growth, not just cost-cutting.
Julien Vadrot, 18, in his last year in high school, said he voted for Sarkozy "because he seems the best in this crisis. For five years, he held the country together ... and kept the country standing better than the other (countries)" like Spain, Portugal or Italy, he said. "It's lost less than the other euro countries."
At a time when voters across Europe have ousted incumbents amid economic woes, a Hollande victory would tilt the continent's political balance to the left even as other leading European nations have governments on the right.
Hollande, who wants to tax high-income earners at 75 percent, has tapped into a fear of the free market that has always held more sway in France than almost anywhere in the West, and has enjoyed a resurgence in the era of Occupy Wall Street and anti-banker backlash.
Foreign policy has barely played a role in this campaign but will be a big part of the next president's job. Candidates of many stripes want to bring France's 3,600 troops home from the NATO-led mission in Afghanistan, and Hollande has vowed a fast timetable: A pullout by the end of this year.
Sarkozy said he wants three debates before the May 6 runoff, on the economy, society and international affairs. But Hollande dismissed that, saying one debate, as had been previously planned, is plenty.
"Because (Sarkozy) is in a very difficult situation he wants to change the rules. But you can't change the rules. When you are a bad student, you get bad grades, you cannot ask to change professors," Hollande said as he left Tulle late Sunday night for Paris.
Sarah DiLorenzo, Elaine Ganley, Jamey Keaten, Thomas Adamson, Cecile Brisson, Sylvie Corbet, Greg Keller, Jonathan Shenfield in Paris, and Masha Macpherson in Tulle contributed to this report.