Independent centre ground candidate Macron and the National Front's Le Pen are expected to make it through to the second round of the election on May 7, according to the polls.
The debate, the first between the five main contenders ahead of a two-round election on April 23 and May 7, could help viewers make up their minds in a French election where almost 40 percent of voters say they are not sure who to back.
By comparison, France hasn't had one.
The debate on TF1 television grew testy when the candidates were asked about migration and Islam.
Markets, surprised by Britain's Brexit vote last June, are nervous about the possibility of a victory by Le Pen, who pledges to take France out of the euro and hold a referendum on European Union membership. In an often heated exchange, the candidates attacked each other's positions, with Emmanuel Macron accusing Marine Le Pen of coming close to defaming him over the subject of money in politics.
But with so many voters undecided and polls showing the abstention rate could be higher than ever in France, the level of uncertainty remains high.
Fillon, accused of paying his wife a generous salary for work she may not have done, has been put under formal investigation, a first for a French presidential candidate. For his part, the far-left candidate Jean-Luc Melenchon mocked much of what Le Pen had to say about immigration and religion.
"The traditional parties, those who have for decades failed to solve yesterday's problems, won't be able to do it tomorrow either", said Macron, who made a name for himself by criticizing sacred cows of the French "social model" such as the 35-hour workweek, iron-clad job protections and jobs for life for civil servants. Francois Fillon characterized German Chancellor Angela Merkel's policy of opening borders to refugees as "bad", while Le Pen said she won't have France turn into a "vague region" of the EU. She also said she would add more police and security forces and expand prison places by 40,000 over five years, moves that Fillon said would add to France's debt burden and lead the country to bankruptcy.