The use of what was particularly important for the advancement of civil rights

The answer is the mobilization masses of people. This refers to deployment of civilian population as part of contentious politics. Mass mobilization is frequently used by grassroots-based social movements, including revolutionary movements, but can also turn into a tool of elites and the state itself. The procedure usually takes the form of large public gatherings such as mass meetings, marches, processions, parades,  and demonstrations. Those assemblies usually are part of a protest action.