This paper examines the diffusion of activism in post-colonial Hong Kong through the lens of the political regime and eventful analysis. It first reveals the institutional foundations of the hybrid regime that allowed the creation of a nascent movement society. It then explains how the historic 1 July rally in 2003 and a series of critical events since 2006 have led to a shift in scale and the public staging of street politics. A time-series analysis and onsite survey further capture the dynamics that spawned the collective recognition of grievances and reduced participation costs, leading to the Umbrella Movement. While the spontaneous, voluntary and decentralized organizational structure sustained protest momentum, the regime has adopted hybrid strategies to counter-mobilize bottom-up activism. The result is widening contention between the state and civil society and within civil society, or the coexistence of regime instability and regime longevity, a trend that is increasingly common in hybrid regimes encountering mass protests.