Hollande and Sarkozy will contend in a runoff election on May 6.
The result of Sunday's vote marks the first time in the history of the French Fifth Republic that a sitting president is not in the lead after the first round of voting.
Eighty percent of eligible voters went to the polls, one of the highest turnouts ever in a French presidential election, according to data from the country's Interior Ministry.
The far-right, anti-immigration National Front failed to repeat its exploit of 1995, when party founder Jean-Marie Le Pen managed to make it to the runoff against Jacques Chirac. Still, Le Pen's daughter Marine carried 20 percent of the vote, the most ever for the National Front in a presidential election, after a campaign based on defending French values against what the party called a tide of immigrants.
Behind her were Jean-Luc Melenchon of the Left Front, who was fourth with 11.7 percent of the vote, and centrist Francois Bayrou, who was fifth with 8.5 percent.
Melenchon's result, while less than the 15 percent he was credited with by pollsters before the vote, is especially important as his votes are likely to flow to Hollande in the second round. Speaking at his headquarters after the exit polls, Melenchon emphatically asked his supporters to vote against Sarkozy by supporting Hollande without asking anything in return.
Hollande and Sarkozy will begin on Monday to court voters who chose losing candidates to secure their support in the runoff election.
If Sarkozy were defeated by Hollande in the runoff, he would become the first French president to fail to get re-elected since Valery Giscard d'Estaing was defeated in 1981 by the Socialist Francois Mitterrand.
Fringe candidates rounded out the results. Environmentalist Eva Joly won 2 percent of the vote; the anti-euro Nicolas Dupont-Aignan took 1.8 percent; Philippe Poutou received 1.2 percent on a far-left, pro-trade union platform; and the 32-year-old Communist professor Nathalie Arthaud garnered 0.7 percent.
Joly also endorsed Hollande immediately after the exit polls came out -- asking her supporters to vote for the Socialist and "do everything to make our country leave Sarkozyism behind".
According to calculations by the conservative Parisian daily Le Figaro based on exit polls, a hypothetical left-wing bloc would get 43.2 percent of the vote in the runoff, by adding the totals of all five leftist and center-left candidates (Hollande, Joly, Melenchon, Poutou, and Arthaud).
Sarkozy could theoretically count on 37.6 percent by adding the Bayrou and Dupont-Aignan vote totals to the president's own Union for a Popular Movement. The calculation does not take into account the National Front.
Winning in the runoff does not require reaching an absolute majority of the vote; a plurality is the sole requirement to become president.