Since the early 2000s, the European Commission has put online e-participation and e-government platforms related to various institutional strategies. These participative mechanisms, which are both policy instruments and communication tools for the Commission, are used by activists as political opportunities to make their voices heard by the European institutions. In this paper, we describehow these activists use Web technical resources, and foremost hyperlinks, to mobilize individuals and run online collective actions in order to make their causes public on European scenes. We show that hyperlinks can be considered as aggregative mechanisms which allow individuals to gather around a common project, and that these mobilization practices lead to the constitution of strategic public spheres on the Web.