I deal with this kind of problem all the time. We have a large, trained german shepherd who is a total sweetheart with family but can be mean as hell with other dogs. We never let her off leash in the city but not everyone here with a dog is that considerate. (Leashes are required by law here.)

At least once a week, someone's dog run up to ours, presumably to play. The ensuing growling, barking, and rising hackles generally scares them off. Now and then, however, another dog doesn't back down and then all hell breaks loose. I do try to restrain my dog, but if the other dog clearly means business, I will absolutely let my dog defend herself. Unfortunately, this has generally meant the other dog will get hurt in someway unless its owner appears on the scene quickly. This is really a shame, because it's not the dog's fault that its owner is irresponsible.

All that being said, the worst place for your hands in a dog fight is anywhere near the business end and all those sharp pointy teeth. The first thing you should do once you grab your dog's collar is attach the leash, which gives you some control at a distance and gives your dog some room to maneuvere and posture. This display alone may ward off an attack, and even if it doesn't, it lets your dog protect itself and keeps you safe as well.

Last point -- one person alone cannot break up a real dog fight, so don't even try. You need both owners involved, preferably each pulling as hard a possible on a leash or whatever it takes to get their dogs to disengage. If the other owner isn't around, then I would join my dog in fighting off the stray, which is an entirely different thing than separating two combating dogs. If this were a more urgent problem for us, I would carry a large stick or pepper spray specifically formulated for canine usage, but I have no idea what's legal where you live.