Collective identity is a key catalyst of protest mobilization. How does collective identity come into existence among strangers with diverse backgrounds, especially in movements without a centralized leadership? Although collective identity is often seen as something constructed by movement organizations or out of established networks, we describe a more bottom-up and decentralized process in which movement collective identity is created through the horizontal mobilization of intermediate identities, which leverage pre-existing social identifications to induce commitment among individuals. Focusing on Hong Kong's Anti-Extradition Bill Movement of 2019, we argue that online petitions against the controversial bill created intermediate group identities among myriad social groups, such as alumni, professions, hobby groups, and residential communities. These intermediate identities provided rich discursive resources for previously disconnected individuals to collectively perceive the threat of the bill and see the obligation to act, which, in turn, shaped a strong collective identity early on in the protests. Our findings may help contribute to a more nuanced understanding of collective identity formation in contemporary leaderless movements
