Barcelona en Comú, an emerging grassroots movement-party, won the 2015 Barcelona City Council election. This candidacy was devised by activists involved in the 15M movement in order to turn citizen outrage into political change. On the one hand, the 15M movement is based on a decentralized structure. On the other hand, political science literature postulates that parties historically develop oligarchical leadership structures. This tension motivates us to examine whether Barcelona en Comú preserved a decentralizated structure or adopted a conventional centralized organization. In this article we analyse the Twitter networks of the parties that ran for this election by measuring their hierarchical structure, information efficiency and social resilience. Our results show that in Barcelona en Comú two well-defined groups co-exist: a cluster dominated by the leader and the collective accounts, and another cluster formed by the movement activists. While the former group is highly centralized like the other major parties, the latter one stands out for its decentralized, cohesive and resilient structure.