As the others said, stop, so that you are not an object to be chased. A couple of "dog language" things: Don't lean forward toward/over the dog, this is an aggressive position which could invite attach from an aggressive dog. Stand straight up tall, a dominant position (which is different than aggressive). Even more effective than yelling "NO" is to growl low and loud at the dog. Think about the noises dogs make when they are fighting, or even the little grunts an older dog makes at a puppy who is annoying it. Its a deep gutteral sound that means "NO" in dog language. If you have water, accompany the growl with a squirt, this mimicks the jaw snap that dogs do to one another, which means "back off". This will work for most dogs. To be honest if a really aggressive or trained attack dog comes at you, there may not be much you can do. And really, as the guardian of two large dogs, if one of them ran up to someone aggressively and the person kicked them, I would totally understand. Don't feel bad, it's a self preservation instinct.