We have a Borzoi, which is a Russian Wolfhound. Looks like a big hairy, greyhound and weighs a bit over 100 lbs now. My oldest daughter was walking it a couple of year ago, it saw another dog next to a house and lunged towards it to check it out. My daughter said the other dog got scared, ran, and in the process of running to it's front porch it scraped itself on a railing or something. She got our dog back on it's leash, talked to the owners, and gave them our address. The owners called and said that they were going to call the police as they said that our dog attacked theirs and bit it, and demanded that we pay the vet bill. I went over to look at their dog, said that they could certainly call the police but it was obvious that it wasn't a bite, especially considering how large our dog's mouth is, our dog's tempermant, and my daughter's observations. I did say though that it would be fair for us to pay the vet bill ($70 or so) as my daughter wasn't able to keep our dog under control. They calmed down a bit, and accepted the offer.
On a walk awhile back when our son wasn't as big our dog lunged after a cat, pulled our son down and dragged him 10 to 15 ft across a muddy lawn. Our son held him though :^)
As a pup our dog could chew thru a nylon leash or 1/4in rope in less than 1/2 minute when it wanted to, so I made the leashes out of plastic covered steel cable and made T handles out of sections of broomstick with a an eye bolt to make it easier to hold. Works well, as the leash has a brass snap at each end and can be attached to or looped around objects.
Pepper spray can work well, but with with strong, aggressive dogs like pitbulls, Rots, etc., you'll have to accept possibly having to beat them senseless with a bat, heavy walking stick, etc., or shoot them.