Group-based norms regarding political participation are not static; they are malleable attributes affected by police–protest dynamics in the social movement context. This study used a comprehensive and authoritative sampling framework to collect the views of Hong Kong residents at the end of the anti-extradition bill movement (n = 1,000; 53.6% female; mean age = 46.4, range 18–86) and evaluated them using the Elaborated Social Identity Model. The results revealed a full mediational pathway from police action perceived as indiscriminate that facilitated the public’s identification with the militant protesters and bolstered public support for the militant protest. Our findings provide an in-depth understanding of why the public supported violent protest and criminal vandalism in the movement by adopting the interactive arena between police and protesters as the entry point and the development of shared identity as the process. Our findings highlight crowd management as a complex issue in relational dynamics rather than a purely logistical challenge. Copyright © 2021 The Author(s). Published by Oxford University Press. All rights reserved.