Social theory implies that a rise in the expectation that many will participate in collective action can make participation in the action widely rational, giving rise to a ‘self-fulfilling prophecy’. I address this classic, yet understudied, proposition by surveying participation in a demonstration that the ‘Panama Papers Leak’ triggered in Iceland in 2016. The demonstration was preceded by a sudden rise of large-turnout expectations, and attracted one-fifth of an urban population, allowing me to obtain event-specific, population-representative survey measures of the focal constructs (N = 821). The findings support hypotheses about the role of large-turnout expectations in collective action. They confirm that protest support (i.e. the value placed in the goal of the collective action) both raises large-turnout expectations and moderates their effects on protest participation. In fact, large-turnout expectations were associated with participation only if individuals supported the protest. Also, the findings imply that large-protest expectations trigger interpersonal relational dynamics that further motivate participation. The study thus supports and yet qualifies the role of the self-fulfilling prophecy in collective action.