Since the corruption scandal against him came to light at the end of January, Fillon has been regularly greeted at campaign stops with banging pots and other gestures of disdain.
A demonstrator pelted French conservative presidential candidate Francois Fillon with flour as he arrived for a campaign rally in Strasbourg on Thursday.
Protests have hit the former prime minister's struggling campaign after accusations he paid his family members for work they allegedly did not do.
Successive disclosures and allegations in the scandal have prevented Fillon from campaigning quietly and overshadowed his proposals.
Video footage from French broadcaster BFMTV showed a young man wearing a T-shirt marked "Students with Fillon" throwing flour at the former prime. Fillon's face, shoulders and chest were covered in white powder.
Addressing supporters at the Strasbourg rally, Fillon said the incident was more evidence that he is "the target of a harassment" campaign, but added: "I hope at least that the flour was French."
Opinion polls show Fillon trailing in third place behind far-right leader Marine Le Pen and independent centrist Emmanuel Macron in the first round of the country's upcoming election.
The two-stage election will be held on April 23 and May 7.