This sort of situation has happened to me and Fraser way too frequently. Dogs in most locations here are supposed to be on leash, but monitoring is slack. I wouldn't even know who to phone. Just the other day as we were finished our walk and heading back to the car, an off leash young boxer/pit(?) mix ran up to Fraser and loomed over him, hackles up, stiff body posture, tail high, staring. Fraser went small and sat down and did all the calming behaviours. I saw the owner and called out to him several times to call off his dog, but either he was deaf (old fellow) or couldn't care less. Eventually I was able to lead Fraser away, but I didn't want to interrupt the early part of the interaction in case it triggered the other dog since it was clearly being dominant. What drives me mad is owners always say, 'he/she's friendly' right up to the moment that the dog makes an aggressive move and then it's, 'oh, Fluffy, be nice' as if it's a naught child. Also, I don't whether Fraser is going to act defensively to another dog's behaviour and set off a brawl. When I was passing the old fellow, I said, 'your dog might be friendly, but I can't account for how my dog is going to react to yours rushing up." He said, 'oh, I didn't think of that.
I empathize with your frustration.