Collective identity has been identified as one of the most important factors explaining protest mobilization and social change. However, understanding the process of identity formation processes under turbulent and contested contexts that evolve rapidly has not received enough attention. This paper studies how group identities evolved during the protests against attacks on Palestine in the period 2023-2024. Computational text analysis is used to extract claims that were voiced by protesters in multiple events throughout Europe and the US, as represented in the media and on social media. Network analysis of the semantic relations between claims provides evidence for how demonstrators from very diverse backgrounds, contexts, and types of movements converge, and how they define group boundaries in response to salient events and the reactions of political elites and organizations. Cognitive and emotional contents in protesters’ claims show that the public expressions of anger and moral outrage were central in explaining how symbols and perceptions were internalized across national frontiers and pre-existing identities. Evidence on the central role played by students in university protests also highlights the importance of commitment and endurance in rapidly building the cognitive contents of social identities. These results shed light on the dynamics of social identity formation processes and the restructuring of group alignments in times of major social shocks.