Actors Christoph Waltz and Daniel Auteuil were among guests being interviewed on the Canal Plus program "Le Grand Journal" when what sounded like gunfire rang out.
Television footage showed Auteuil and Academy Award winner Waltz—a member of the Cannes festival jury—scrambling from the seaside stage with dozens of others as a voice said "there's someone shooting."
When transmission resumed minutes later, host Michel Denisot said the noise had been caused by blanks. There were no reports of injuries.
Photographs showed a man being wrestled to the ground by police near the stage beside Cannes' seafront promenade, the Croisette.
Local newspaper Nice-Matin reported that a man was detained by police with a starter's pistol, a pocket knife and a plastic grenade.
Police could not immediately be reached for comment.