interjection
Objection!; I object!
A forceful interjection used to raise an objection, often in formal or dramatic contexts such as courtrooms, debates, or when strongly disagreeing with a statement.
裁判長、異議あり！
Your Honor, objection!
その意見には異議ありだ。
I object to that opinion.
反対 is a general noun/suru-verb meaning 'opposition' or 'to oppose', while 異議あり is a fixed interjection specifically used to voice an objection.
A set phrase combining 異議 (objection) and あり (exists). The expression is strongly associated with courtroom dramas and formal debate.