I had a similar event with a mutt once, outweighed me, and was mean. Tried to latch on to my forearm, but I turned away just in time, and grabbed his jaws and held them closed as I wrestled him down to the ground. Held him for a good ten minutes, and he was growling, and flailing trying to get loose, but I held on tight. His owner heard him, and came running out with a leash. Once he had mutt secured, I let go. Luckily, dogs have very little strength opening their mouths.