In Utah, I do not know the answer of whether it would be a crime. It is not clear. In Georgia? No crime. A third person is privileged to stop an unlawful arrest in Georgia against another, which the law here sees as nothing more than the crime of battery by the officer.
Before you start making sarcastic comments, you should know I have stopped an unlawful arrest in Georgia with the threat of force. The officer backed down at the last second, but I was fully prepared to do what I had to do to stop him. Since we were at a police station, he had all the backup he could want if needed.