。
The police squad ended up in a brawl with the demonstrators.
Literal
Police-squad [topic-は] demo-squad [with-と] brawl [into-に] became.
~になる 'became X' is the standard pattern for 'ended up in state X.' 乱闘 'melee, brawl' is a noun, so it's linked by に as the resulting state. 警官隊 and デモ隊 both use the ~隊 suffix for organized groups — a productive pattern also seen in 楽隊 'band,' 自衛隊 'self-defense force,' 救助隊 'rescue squad.' と marks the partner in the reciprocal action ('brawled with').