The 15M demonstration (the origin of the indignados movement in Spain and the seed of the occupy mobilizations) presents some outstanding characteristics that defy the established principles of the collective action paradigm. This article develops some observable implications of the concept of connective action and tests them against the case of the 15M demonstration. Cases of self-organized connective action networks are expected to be different from traditional collective action cases with regard to the characteristics of the organizations involved, the prevalent mobilization channels and the characteristics of participants. Based on a comparative analysis of data gathered from participants and organizations in nine demonstrations held in Spain between 2010 and 2011, relevant and significant differences were found in the characteristics of the 15M staging organizations (recently created, without formal membership and mainly online presence), the main mobilization channels (personal contact and online social networks rather than co-members or broadcast media), and participants (younger, more educated and less politically involved). These findings help to understand the large turnout figures of this movement and have important implications for the mobilization potential of social networks, particularly as it can affect the political participation of the less-involved citizens. © 2013 Taylor & Francis.