If it has your hand, grip the lower jaw (you've already been bitten anyway). The dog will lose nearly all of its bite strength (I've been through this before, it's true) and will be lucky if it can even break your skin. Just make sure you use your other arm and legs to stabilize the dog, which at that point will panic due to this.

I hate hate HATE killing dogs for any reason, but after you've stabilized the dog's mouth, proceed to use your spare arm to choke it out. Be vicious, be savage, and don't get all soft when it starts to whine.

I feel sick just typing this...

Run the poor thing off if you can... it's likely just hungry due to no food, thanks to urbanization and habitat depletion.