This sounds exactly like a situation I found myself in in September 07.
Walking home from school with a friend. Four children in total. I had my two dogs, both on leads - labrador and JRT. Friend had her Springer on a lead.
Rottweiler came bounding across a main road. Laid into my labrador. The Rottweiler was so big and strong I could not pull her off my dog. I screamed and shouted and in the end I had to kick the dog off. The owner (young lad - covered in earrings, tattoos and busy with a fag) came out and started f-ing at me telling me it was my fault? My labrador got back on to her feet and held her back leg up. Whilst comforting the kids, my dogs, and getting myself to calm down the owner came out again and this time I had a go at him for being completely unreasonable for swearing at me, told him he should keep his dog under control, or better still, secured in his garden/tied up. He then showed his fist to me so I told him I'd send the Police round to him.
I walked home with crying children, a dog limping and me very shaken up.
Rung DH who was on his way home - gave him the address - he went and saw the guy and told him if he ever threatened me again he'd call the Police and to keep his dog under control.
His reply was that the dog was only 9mths old (by god it was big for 9mths) and in season - dh told him all the more reason to keep it secure so not to be mated etc. The owner also added he has a problem with his temper and he apologised for his threatening behaviour.
I ended up in the vets that evening with my dog who had pulled a muscle in her back leg - she is 12yrs old - she has been on pain relief ever since.
I'm not labelling rottweilers as nasty vicious dogs but if they fall into the wrong hands they are a breed, imo, that can easily become that way. Same applies to pretty much any dog - given the right care, attention, training etc any dog, I think, has the potential to be lovely.