The aim for centrist Emmanuel Macron and his far-right rival Marine Le Pen is to win the votes of large numbers of undecided or reluctant voters.
While Mr Macron is well ahead in the polls, his lead has narrowed and an estimated 18% of voters are undecided.
For the first time, neither candidate is from a mainstream French party.
Although Marine Le Pen's father qualified in 2002 for the run-off as head of her party, the National Front (FN), his rival and eventual winner, Jacques Chirac, refused to take part in a debate because of the FN leader's extremist views.
The 2017 candidates began their two-hour-20-minute debate by attacking each other. Ms Le Pen called Mr Macron a shameless "darling of the system" while Mr Macron said his opponent was cloaked in a "spirit of defeat".