If push comes to shove, strip down your overcoat/shirt, whatever cloth you have on you, entwine your fist with it, covering your wrist. Use it as bait for the dog to bite on it, it will instinctively do that. Same goes with sticks - if you have a long stick, stick it out and the dog will go for it first in order to disarm you. Kick it while you're in either situation. Rinse and repeat, dog will give up eventually.
If kicks do not work, use melee weapon as
>>1020699 suggested.
When defeated, dogs will become more aware, just like humans, thus lowering the chance of aggressive behavior.
In packs, however, the bastards tend to get cocky. I'm afraid martial law is out of the question in this one - here we just gather the local hunters and hunt the pack. Dogs are animals. An aggressive dog is no different than a starved wolf.
Hey, pepper spray ain't bad against packs as well. Who the fuck is gonna come and nag you for spraying some animals for attacking you?
Stay safe from these cunts