They decided to protest non-violently by sitting in silence and giving speeches to advocate for change.

non-violently 是由前缀 non-（表示“非、不”）和形容词 violent（暴力的）加上副词后缀 -ly 构成的副词。violent 来自名词 violence（暴力），意思是“暴力的”。non-violently 合起来表示“以非暴力的方式”。