noun
riot policeman; riot police officer
Refers to an individual member of the riot police (機動隊). Used in news and formal contexts.
デモ会場には多くの機動隊員が配置された。
Many riot police officers were deployed at the demonstration site.
機動隊 is the riot police unit as a whole, while 機動隊員 refers to an individual member of that unit.
Compound of 機動 (mobile, maneuver) + 隊 (squad, unit) + 員 (member). The term is a straightforward modern formation.