French President Nicolas Sarkozy and his Socialist challenger, Francois Hollande, are holding their only TV debate of the election campaign.
Correspondents say it is Mr Sarkozy's last chance to regain the initiative in the race after losing to Mr Hollande in the first round last month.
Opinion polls suggest Mr Hollande has a lead of seven percentage points.
However, the president is seen as a formidable debater while his rival lacks experience of government office.
The debate is being broadcast live by several channels and is expected to have a maximum running time of two-and-a-half hours.
About a third of France's 63 million people were set to watch the event, billed variously by newspapers as The Last Duel and The Final Confrontation.
'Pushed to the limits'
Earlier, some good news arrived for the Sarkozy campaign in the form of two new opinion polls suggesting he had cut his rival's lead by one point - but the Ifop and BVA polls still had him seven points behind.
French voters interviewed anonymously by the Associated Press news agency welcomed the prospect of the debate.
"Many people still don't know who to vote for," said one. "More than ever this debate could be decisive."
"It's always good to see them facing each other," said another.
"It could reveal a certain number of things. Francois Hollande hasn't said much so far, it will be interesting to see him pushed to the limits and say what he plans to do."
Mr Sarkozy has attacked Mr Hollande for refusing to hold three election debates instead of one but there has been just one debate per presidential election since 1974, apart from in 2002 when Jacques Chirac refused to debate with the far right's Jean-Marie Le Pen.
A key moment in Mr Sarkozy's 2007 debate with Segolene Royal - Mr Hollande's former partner and mother of his children - came when he told the Socialist candidate to "calm down". She repeatedly refused to do so, saying some anger was "perfectly healthy".
Wednesday's showdown is being presented by two French TV anchors, Laurence Ferrari of TF1 et David Pujadas of France 2.