First thing to do when you see a strange dog is to NOT ACT TENSE. No matter how nervous you *want* to become, you can NOT give the outward impression that you are apprehensive. This will just backtrack your aggressive dog's progress. Second thing is to place yourself between the strange dog and your own dog. I usually don't have to encounter these situations, but I would place my fist out and say "NO/STOP" firmly. If the dog doesn't stop in his/her tracks, I'd grab my handy Bitter Apple Spray and spray it at the dog's front paws. If push comes to shove, I'd spray the Bitter Apple at the dog's mouth. Once the dog backs off, I'd personally go back home and try another route or walk on if you wish.

Above all, I agree with the others in that you should contact AC so that the leash law is either more strictly enforced and/or revised so that it more severely penalizes offenders. Most people think they're doing Fido a favor by letting him run off lead, hence, gaining exercise. They just need a nudge in the right direction. I wish you the best of luck in solving this problem!