Royal, aiming to be France's first woman president, took to the stage in the southern city of Toulouse, alongside Spain's socialist prime minister, and called on voters to back her and defend French democracy.
"Democracy needs you. The [French] Republic needs you because today it is threatened by ... fracture," she told the flag-waving crowd.
If, as expected, none of the 12 candidates wins an absolute majority at the April 22 vote, the top two contenders will go forward to a run-off ballot on May 6.
Recent polls have suggested that Sarkozy and Royal will see off a challenge from centrist Francois Bayrou and the far-right leader Jean-Marie Le Pen to set up a classic left-right second-round battle.
|Defaced Sarkozy posters in Paris [Reuters]|
The same polls have also indicated that Sarkozy's long-standing lead over Royal is eroding, with a CSA survey released on Thursday saying the two were level-pegging before a probable second-round run-off.
However, polls have got it wrong in the past. Le Pen stunned France in 2002 by knocking Lionel Jospin, the socialist candidate, out of the race, before going on to be crushed by Jacques Chirac in the run-off.
Sarkozy has come under fierce attack from all sides in recent weeks, with opponents of all persuasions portraying him as a dangerous, divisive force in French politics.
He told supporters in Marseilles: "When candidates have no ideas, no arguments, no convictions, when they believe in nothing and don't work, they have no other option than insult, lies and insinuation."
Voice of moderation
The election campaign has run against a background of worries over jobs, immigration and security, but increasingly focused on the personalities of the main candidates.
Mar 16: Deadline for presidential candidates to get at least 500 mayors or other elected representatives to sponsor them. Without these, candidates cannot stand.
Mar 19/20: Constitutional Court examines proposals, decides list of official candidates.
Apr 9-20: Official campaign period.
Apr 21: First round presidential voting in overseas territories and departments. Guadeloupe, Martinique, Guyana, Saint-Pierre-et-Miquelon and French Polynesia.
Apr 22: First round of voting on mainland France, the Indian Ocean islands of Reunion and Mayotte and in New Caledonia.
May 5: Second round of overseas voting.
May 6: Second round of mainland voting.
May 17: End of President Chirac's mandate.
Jun 10: First round of parliamentary voting.
Jun 17: Second parliamentary round.
Jun 19: End outgoing parliament.
Bayrou has portrayed himself as a voice of moderation and urged voters to rebel against years of political ping-pong between the right and left.
But he suffered a setback on Thursday when the influential Le Monde daily called on the French to vote either Royal or Sarkozy, saying they had the most coherent programmes and the strongest teams for eventual government.
He denounced the editorial as "arrogant" and "disrespectful".
"Who are these people, where is their patent on democracy?" Bayrou said during a rally near his home in southwest France.