i think probably workforce is better, but they are talking about organizations

personas sobre las cuales no se duda, son" incontestables, pas discutés"

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Note added at 2 horas (2008-09-16 00:43:17 GMT)
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responsables de la movilisation

If you want something in native (UK) English, you should consider rephrasing the entire sentence:

Depuis lors, les Organisations de la Société Civile ont été les cadres incontestés de mobilisation des masses populaires.
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Ever since then the masses have channelled their actions systematically through Civil Society organizations.

Civil society - Wikipedia, the free encyclopedia - [ Traducir esta página ]Civil society is composed of the totality of voluntary civic and social organizations and institutions that form the basis of a functioning society as ...
en.wikipedia.org/wiki/Civil_society - 75k - En caché - Páginas similares
What is civil society? - [ Traducir esta página ]What is civil society? Civil society is a concept located strategically at the cross-section of important strands of intellectual developments in the social ...
www.lse.ac.uk/collections/CCS/what_is_civil_society.htm - 9k - En caché - Páginas similares

Its simple no need to explain.