But National Front champion Marine Le Pen won about 18 per cent, beating the party's previous record of 17 per cent set in 2002 by her firebrand father, Jean-Marie Le Pen.
Among the other major candidates, Jean-Luc Melenchon of the Communist-backed Left Front won about 11 per cent and centrist Francois Bayrou about 9 per cent.
Turnout was higher than expected, at more than 80 per cent, according to estimates.
In their first pronouncements, the frontrunners made eyes at the vast pool of votes that will decide the presidency.
"I am the candidate of national unity," Hollande said. "Unity must be as broad as possible."
Sarkozy said: "I am asking all French people who love the nation above all else or above private interest to come together and join me." He used the word "patrie", or homeland, which is commonly used by the far right.
By some standards, the French President is the most powerful figure in the world.
Named for a five-year term, the head of state appoints the Prime Minister and Cabinet, can sack Parliament and call new elections and takes charge of foreign affairs and defence, including France's nuclear weapons.
In 2007, Sarkozy romped to victory over Segolene Royal, who is the mother of Hollande's four children and formed with him a power duo in the Socialist Party before they split up that year.
Pre-election estimates suggested Hollande could inflict a stinging revenge, by as much as seven percentage points, next month.
But yesterday's results muddies the picture.
Hollande will have to secure the far-left vote and Sarkozy will have to pitch to the far right - and both will have to woo Bayrou's centrists - in order to make it over the finishing line.
The next two weeks will be marked by far-right and far-left rallies on May 2, and on May 3 by the first TV confrontation between Hollande and Sarkozy.
The President, sensing that hand-to-hand confrontation offers him his only chance of KO, yesterday demanded three one-on-one debates, but this was rejected by the Socialists. "Nothing is over. There is absolutely everything still to play for," said Foreign Minister Alain Juppe, Sarkozy's right-hand man.
Sarkozy's big handicaps have been his sharp tongue, his flashy style and unpopular, if mild, reforms that have failed to fix France's economy.
His campaign has been pitching on an image of a "can-do" President, whose energy had saved the euro during the Greek debt crisis.
His talk of curbs on immigration and crimping Europe's borderless Schengen arrangement has so far failed to win over the National Front supporters. The Front's score was even higher than in 2002, when Jean-Marie Le Pen beat the Socialist candidate, Lionel Jospin, to contest a runoff against Jacques Chirac.
"The battle of France has only just begun. Whatever happens in the next two weeks, nothing will be the same as before," Marine Le Pen told cheering supporters.
The "elitist" system of mainstream parties, corrupt politicians, meddling by Brussels and bankers was tottering, she claimed. Melenchon called on supporters to back Hollande in the second round.
Bayrou said he would "reflect for several days" before giving his recommendation, a delay that opens the way for horsetrading. Le Pen said she would announce her decision on May 2, when the Front commemorates Joan of Arc, France's patron saint.
Hollande, 57, stepped from the wings to become the Socialists' stand-in candidate after the favourite, Dominique Strauss-Kahn, was sidelined by a sex scandal.
Working the crowds, he has scored well as a stable and affable alternative to the high-octane, yo-yoing Sarkozy and by appealing to the strong egalitarian streak in French politics.
His Achilles' heel, though, is his grey style and lack of experience in government, for he has been a party functionary all his career. Critics also find flaws in his economic programme of high taxation and government spending, a strategy they predict would be disastrous at a time when European countries are tightening belts and reducing deficits.
