This paper examines the roles of professional networks in mass protests. The extensive participation of professionals in the anti-extradition movement in Hong Kong, using their professional expertise, specialized networks and institutional positions, constituted a novel form of collective action. Based on framing analysis of sectoral petitions and interviews with participating professionals, this paper shows that state–corporatist arrangements and social movement abeyance structures laid the foundation for sectoral mobilization in the anti-extradition movement. It reveals the conditions under which professional activism can move beyond individual practices and overcome organizational barriers to generating resources to sustain a mass movement. Perceived threats to the professional ethos triggered cross-sectoral participation. Sectoral mobilization modes and levels were contingent on an array of institutional constraints, informal networks and conjunctural events that made for widespread and legitimate professional involvement in a networked movement.