MoM- I had a similar experience in the park about 6 months ago. There was a woman with a black dog (looked huge to me and huge compared to my DS who had just learned to walk). I saw the dog run straight towards DS from a distance, and my first reaction was to pick DS up before the dog came too close.
The dog wouldn't give up, he started to jump up and down trying to touch the baby. Ok, I am not a dog lover and I don't own one, but the dog looked aggressive to me. A few minutes later, as I had started to walk away in the other direction, the dog and the woman owner started to walk towards us AGAIN and the dog started running towards DS again.
The woman made no attempt whatsover to control her dog, despite the fact that she could see I was quite upset the first time.
Instead, when I shouted towards DH to pick DS up again before the dog reached him, the stupid woman turned round and shouted at the top of her voice "Oh, stop it you paranoid woman, I am training this dog and she is used to being around children" to which I answered, "Well don't train your dog on my son, can't you see how small he is" ? I was furious She didn't even bother to apologise.
Had similar situation with another woman, whose dog was massive, but wasn't half as aggressive looking, and when the dog approached DS, I immediatley picked up DS. The owner came up to me, apologised several times, and said she hoped the dog hadn't scared my son. Now, that's what I call manners !!