adopted dog gets frightened and barks at people

Question
2 months ago we have adopted a female dog, estimated to be 18 months old. We live in Greece and assume she was a street dog (there are many here) and that in her prior life she had been hurt by people. The vet thinks she is a mutt- german shepherd and golden retriever. She is generally almost too submissive (getting on her back instantly) and eager to please us. We have been trying to train her using positive rewards and she is learning to walk on the leash without pulling, to come, and to sit. She is generally very agreeable. The difficulty is her fear of people- primarily men. We live in a very populated area and I sometimes need to walk her in dense areas full of tourists. When she is frightened she will suddenly act aggressive and bark most ferociously. This has the effect of scaring the person she has become frightened of- which increases her own fear. A cycle that escalates. I'm not sure how to stop this. We believe that she had been hit by people in the past because she gets very frightened by people who raise their hands over her head- something people often do when they want to pet the top of her head... And because she is generally a lovely dog, strangers are often coming without asking to pet her. Most of the time this is ok, but if she senses they are fearful, or sometimes for unknown reasons with men, she will just begin her ferocious barking. This can be intimidating to people because she is a medium-large sized dog. So far she has not bit anyone but I am concerned and I feel like I must be constantly vigilant. Any advice would be very much appreciated. Thanks.

Answer
Your dog needs more socialization. You're going to need treats and some volunteers. It will also help to teach your dog some obedience and training her to do specific things when she meets someone on the street, like to sit and offer her paw. If you teach her that this is the appropriate way to greet new people, then this is what she will do and it will discourage people from reaching over her head, which she clearly does not like (this is common, by the way, the right way to pet a dog is on the chest, not by reaching over the head as that is a very threatening gesture)

First, invite lots of men over to see your dog in the comfort of her home. Have her sit and offer them her paw and have them speak gently to her, praise her, give her treats and scratch her chest and side (not reaching over her head)
Next, invite lots of men to meet you in the park. As you meet each person, have them do the same.

If you make your dog to sit and offer her paw every time you meet a new person and that person then reacts with delight (they will take her paw and laugh at her cleverness which will make her feel great) your dog will learn to automatically sit and offer her paw every time a new person approaches.

Be very careful with children though. My dog tells me that the proper way for a child to pet a dog is on his side. When you have a medium to large size dog their face can be exactly level with a child's face when they are sitting and the dog is standing and if the child stares into the dog's face, the dog can see this as very threatening and confusing (especially if the kid then reaches over the dog's head, or worse, hugs it!) Encourage children to take their handshake quickly then move to the dog's side and pet her back and shoulders. And, of course, warn people never to reach over your dog's head. People shouldn't do that anyway. It's bad manners in dog language.