  Barcelona en Com\'u, 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 Com\'u 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 Com\'u 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.
