The woman -- who later identified herself to TV cameras as 45-year-old Claire Seguin -- ran up to the podium where Socialist Francois Hollande stood to sign a "social contract" in favor of housing for all.
Hollande, who has consistently led polls, well ahead of President Nicolas Sarkozy, remained calm throughout the incident, though his glasses, hair and suit were covered in white.
He was hustled off the stage, and when he spoke to reporters afterward, he stayed on point, discussing the importance of housing reform.
Body guards who jumped in to protect Hollande also took a dusting.
The guards also immobilized Seguin and then carried her off the stage by her hands and feet.
A police official later confirmed that she was carrying a kilogram of flour at the time and is in police custody. The official spoke on condition of anonymity, citing policy.
The guards who carried her off the stage later let her speak briefly to a few journalists before taking her away. She shouted into their cameras that she had been the victim of an unspecified injustice and that the Socialists were trying to kill her. She directed them to her blog.
On the blog, which includes an open letter to Sarkozy, Seguin describes being harassed by superiors when she was a teacher and claims that she is the victim of a concerted campaign to defame her and that her life has been threatened and her privacy violated. She says she has filed numerous legal complaints, but none has been acted on.
Najat Vallaud-Belkacem, a spokeswoman for Hollande, told French television channel BFM that the incident was isolated and committed by a woman who appeared unstable.
She said Hollande's security would be gradually increased as the campaign progressed, as it would for all candidates. The election will be held in two rounds in April and May.