I'm not a lawyer, a cop, or a paramedic, just a First Aid instructor, and this is all 20-20 hindsight anyway. However, as I understand it, neither you nor the other paramedic were in uniform or on duty; the police officer was both. Therefore, he is officially in charge. Rather than getting into a machismo contest with him, I think you should have introduced yourself as a qualified paramedic or whatever your qualifications are, and stated "My recommendation is [whatever your recommendation is, e.g. "these patients should remain standing until the ambulance arrives"]" If he chooses to override/ignore your advice, or chooses to believe the other off-duty paramedic, that's his prerogative. (fwiw, before reading this thread, I think I would also have tried to get the two patients to lie down.) Look at it from the cop's point of view - he's probably attended hundreds of accident scenes, he's trained in first aid, at a minimum, he's the only guy there in uniform and he's trying to do his job, and some civilian comes over and tries to make him look bad in front of everyone. How would you react if it were you? Especially when there's another off-duty EMS provider who openly disagrees with you?

Did the other EMS provider have actual work experience? Did he have the same qualifications as you? Did he or the police officer know that you had only just completed your training? (Am I correct in assuming that, if you'd just passed the paramedic's exams, you must have several years of work experience as an EMR or EMT?) If I were in the cop's shoes and I were confronted with conflicting opinions from two equally qualified providers, one of whom worked in the field and the other who had "only just graduated", I know which one I'd be more likely to listen to. (So emphasize your work experience as well as your level of training; e.g. "I'm a paramedic with [name of employer] for the past [n] years." rather than "I just completed my paramedic training this afternoon and you're doing it wrong." which doesn't get across the message "I have 5-10 years of work experience as an EMT".)

Technically, as you were the first on the scene, you are in charge until you voluntarily relinquish control to another equally- or better-qualified bystander, or EMS takes over. From your level of training, you almost certainly know the legalities better than I do. However, the other off-duty EMS (a) may have had more experience than you, (b) may have seen something you missed, or (c) may not have known there was another off-duty paramedic on the scene.

Maybe it would have been better if you'd gone up to him and said "Hi, my name is ...., I'm a paramedic with [name of employer], I'm glad you're here. I have these two patients stabilised, but there are people trapped inside, I could use your help over here." or something like that. I know, I know, easy enough for me to say while I'm sitting here goofing off at work, not so easy when it's dark, cold and wet and there's six injured passengers and two off-duty paramedics who don't know each other, but that's why you asked the question, right? <img src="/images/graemlins/wink.gif" alt="" />

He was trying to help, you were trying to help, the cop was trying to help. If I interpreted your post correctly, it's a shame that you ended up getting into a shouting match in public.

With multiple casualties, I would not have left the scene until the ambulance showed up, I would have made my report to the on-duty paramedics when they showed up, then I would have gone to the zoo. This may be very unfair, but your post makes it sound like you took your ball and went home in a pout because the other kiddies wouldn't play by your rules. I'm sure you were far more concerned about the patients' safety than you were in proving you were right, but that didn't come across clearly in your post.