A woman near us has many dogs and walks them where I walk my gang. When we had our cockers, one of her dogs first attacked Buffy and a month or so later attacked Harriet.
When it pinned Buffy the rest of my dogs ran to help her and it was horrendous - I was trying to unlock it's jaws from my precious girl and the rest of my gang were snarling and trying to get it off her too.
Luckily she wasn't too badly hurt.
Then the same dog attacked Harriet in the same way.
This time I saw red and had a real go at the owner. She assured me it had never done it before - so I reminded her it had done it to Buffy not long before. I told her in no uncertain terms it must be muzzled as it was dangerous.
I walked them at different times and different places for a while - then I saw her again minus said dog.
I asked where it was and it transpired it had been put to sleep after attacking a child

Someone once told me if a dog really locks onto your dog, don't ever pull at the collar as if it works the dog will come away with a mouthful of your dog too - twist the collar thereby strangling the attacking dog to make it let go. She also said as a last resort put your finger up the dog's bum -not nice to think about but let's face it if nothing else was working we would try anything.
Again, I am not advocating that and when my girls were attacked I was too busy trying to get the attacker off without thinking rationally about ways to do it. As other say, it happens so quickly.
This is one of my biggest fears as I know the rest will rush in to try and help and so it could well end up with all of them getting hurt - even Suki who is the biggest wimp in the world and will hide behind me if a dog so much as glances her way, will go to the aid of the others if they are in trouble.