Incumbent President Nicolas Sarkozy repeatedly called Socialist challenger Francois Hollande a "liar" as the on-going prime-time televised TV debate of French presidential race heated up late on Wednesday night.

The frontrunner Hollande accused Sarkozy of dividing the French, while the incumbent Sarkozy alleged that Hollande's economic program could be catastrophic and cause market chaos.

Tensions came frequently in the debate. When Hollande threw out the question why Germany was doing better than France, Sarkozy struck back by saying that Germany had been doing the opposite of the policies proposed by Hollande.

While Hollande vowed to be a president of "justice" that would restore production and jobs and bring the French people together, Sarkozy immediately refuted that it would be too late if voters elected someone from scratch.

The incumbent added that he was unfairly blamed for the country's economic problems after years of crisis, while Hollande could not help complaining that he was blaming everything on crisis.

The presidential debate, a classic arrangement in French politics, kicked off at 9 p.m. local time (GMT 1900) at a studio north of Paris and was agreed by both sides to last no more than two and a half years.

An estimated 20 million French voters were tuning in to watch the so-called "last duel" that was broadcast live on major national TV channels and seen as the climax of presidential campaigns four days before the runoff election.

The two rivals, almost identically wearing dark suit and tie and sitting apart across a table, were expected to continue attacking each other's policies and proposals to the maximum level until almost mid night.

Sarkozy had previously billed this debate as the "moment of truth" in the election race and even pushed for two more debates, only to be turned down by Hollande.

As Hollande has been predicted by all polls to win him comfortably, the incumbent was under huge pressure, pinning almost all his hope on the debate to gain enough momentum and swing Sunday's runoff vote into his favor.

The latest setback for Sarkozy came on Tuesday when far-right leader Marine Le Pen refused to endorse him and announced she would cast a blank vote. But on Tuesday he also staged a huge rally that attracted hundreds of thousands of supporters at the Trocadero square.

Wednesday night's debate was moderated by leading French TV presenters David Pujadas and Laurence Ferrari. Local media said both campaigning teams even agreed on logistical details in advance, from the studio's constant temperature to chairs that could be adjusted for height.

It remains unknown if Sarkozy could seize the last chance to convince more voters in order not to be defeated, while analysts say debates will most likely only confirm voters' established views.

The two candidates, being scrutinized by dozens of TV cameras in the debate from every angle, had a history of several debates against each other but for much lower stakes.