On the 35th edition of the third stage of the Coupe de France selection of domestic races, FDJ-BigMat rider Demare pipped Laurent Mangel of Saur-Sojasun and his FDJ team-mate Mickael Delage.
All three Frenchmen finished in a time of five hours, seven minutes and 20 seconds, with the main group two seconds behind.
Demare, 20, won stage six of the Tour of Qatar, the Le Samyn day race in France and the second stage in the Dreidaagse van West-Vlaanderen, where he also won the green jersey.
2. Laurent Mangel (Fra) Saur - Sojasun
3. Mickael Delage (Fra) FDJ-Big Mat
4. Adrien Petit (Fra) Cofidis, Le Credit En Ligne (0:00:02)
5. Pierrick Fedrigo (Fra) FDJ-Big Mat
6. Romain Feillu (Fra) Vacansoleil-Dcm Pro Cycling Team
7. Jonathan Hivert (Fra) Saur - Sojasun
8. Francesco Lasca (Ita) Caja Rural
9. Nacer Bouhanni (Fra) FDJ-Big Mat
10. Guillaume Blot (Fra) Bretagne - Schuller