While Casualty often has multiple storylines per episode, the most prominent episode directly matching your description is:

Series 29, Episode 9, titled “Entrenched” (originally aired November 1, 2014).

This episode specifically revolves around a local football derby day where a pre-match fight spirals into tragic consequences, flooding the ED with injured, rioting fans. It depicted scenes of extreme tension, violence, and the staff’s struggle to manage the ensuing chaos.

There was also a more recent episode in Series 37, Episode 5, titled “Derby Day” (originally aired September 10, 2022), where an accident at a football stadium floods the ED with rival fans and Stevie steps in to shield a frightened father and daughter.

So, while “Entrenched” is the classic “fight breaks out on football derby day” episode, “Derby Day” in Series 37 also features similar chaos.

Series 29, Episode 9, titled “Entrenched” (originally aired November 1, 2014). This episode specifically revolves around a local football derby day where a pre-match fight spirals into tragic consequences, flooding the ED with injured, rioting fans. It depicted scenes of extreme tension, violence, and the staff’s struggle to manage the ensuing chaos. There was also…