At first denied by the government and made invisible in the mainstream media, the theme of police brutality emerged in December 2018, as part of the Yellow Vests conflict, on social networks, through the broadcasting and discussion of a limited number of viral videos. Is this a mere manifestation of the mechanisms of the proof by image, associated with participatory mediation? This paper intends to analyze the narrative charateristics and media processes at work in specific circumstances to select and establish these videos as reference corpus. By employing the copwatching reading grid, online conversations have built a controversial space based on the denunciation of visible facts, a real alternative to institutional media coverage. The later coverage of police brutality in the mainstream news outlets attests to the fact that the yellow vest crisis has made of social networks the catalyst of public debate today.