The six candidates are front-runner François Hollande, former party leader Martine Aubry, the 2007 presidential candidate Ségolène Royal and three others, Arnaud Montebourg, Manuel Valls and Jean-Michel Baylet.
The six drew lots on Tuesday to decide where they will stand during the debate. By chance, the two front-runners have ended up standing side-by-side in the middle.
The debate, to be shown on France 2 at 8.30pm, will be moderated by three journalists. Each candidate will be able to give a short speech, after which they will be individually questioned. There will then be an open debate between them.
Hollande and Aubry scored 40 percent and 22 percent respectively in the most recent poll published on Monday in newspaper Libération. Ségolène Royal trailed the front two with just 12 percent.
In advance of the contest, Hollande said he wanted a calm debate on Thursday.
"These debates should not push us to confrontation. There is no major contradiction between us. We are in the same party, we voted for the same project and we will be together in 2012," he told Metro newspaper.
While the candidates have maintained an air of forced civility, Hollande's former partner with whom he has four children, Ségolène Royal, has been more confrontational in her remarks.
"François Hollande's weak point is his lack of action," she said recently. "Can the French point to a single thing he has done in thirty years of politics?"
The primaries, as they are being called, will take place in two rounds. The first round of voting will be on October 9th, after which the leading two candidates will go into a final run-off on October 16th. Anyone who pays one euro is eligible to vote in the elections.
The presidential election operates on a similar system of two rounds of voting and is planned for April 22nd and May 6th 2012. Current president Nicolas Sarkozy is expected to run but has not yet made an official declaration.