The diffusion of digital media allows the emergence of new types of relations between grassroots campaigners and organisers. This article presents the results of a comparative qualitative study of two Italian cases of grassroots online participation: a local electoral campaign and a single-issue social movement. The first case is Tell your Milano , a project that took place during the electoral campaign for Mayor of Milan in the spring of 2011. The second case is the Purple People (Popolo Viola), an Italian social movement started in 2009 to demand the resignation of the Prime Minister, Silvio Berlusconi. The article introduces the concept of a grassroots orchestra : a grassroots campaign aimed at a short-term objective, coordinated by a non-grassroots political actor and performed by a community sharing a uniform and coherent context. (c) 2014 Australian Political Studies Association.