Personally, I'd get to know the dog and the dog owner. What kind of dog is it? Is it just a little yapper threatening your ankle - or is it the prize fighter pit bull owned by the next door dope dealer? 999 times out of 1000 you can solve the problem without having to spray the dog or call an 'authority'.
I'm thinking the dog probably see's you as some sort of territorial threat and is just letting you know he's concerned and a little scared of you. Looking the dog in the eye and speaking firmly - rather than reacting in a way that encourages more aggression/barking...is likely to be helpful. Tell the owner about it - and maybe after a friendly chat and petting the dog... you will feel better and the dog will feel better.
I hope you weren't considering a firearm to be used in the situation. :barf:
On the other hand, if it's a mistrained pit bull or an apparant real 'mad dog'
and it's running loose - I'd call Animal Control.
