But off-duty officers are just civilians? They have no added powers to use force against citizens?
That's correct. Police officers can only exercise police powers while on duty, and when they go off shift, they become civilians.
Don't you understand that an off-duty police officer may not be in uniform, that that's illegal? In some jurisdictions an off-duty police officer is expected to carry his gun, but he may not exercise police powers.
In short, when a police officer goes off shift, he loses his police powers. Being a policeman is a job, not an identity.