How is the dog when not on lead with strange humans? MY guess is you have not noticed any fearful or aggressive action when he is off leash. I am going to proceed on that basis but if that is not the case then a different apporach will be needed.
Keep in mind if the dog intent was to cause injury it would not be growling or even snaping these are all means for the dog to comunicate stay away
Actual this attitude is probably particially to blame for the dogs behavior. What you are dealing with is commonly caled on leash agression and the leash and the feed back the dog gest through the leash are contributing factors. Keepin in mind that a leash and or tie out is a contstraint on the dog normal ability to react to fearfull situation. It has no means of escape. So in the face of what it is scared of it resorts to it only alternative.
Canine Concepts
I thinkg the following articles will shed a lot of light on what is going on
AGGRESSION: A Case History with Harry T
Help! My On-Leash Dog Barks and Lunges at Other Dogs!
keep in mind if it is dog or people the cause of the behavior and how to correct the behavior are not different.
Handling On-lead Aggression
Nick Jones of Alpha Dog Behaviour discusses aggression when on the lead, and how to cope with it
includes video
resource
FEISTY FIDO - HELP FOR THE LEASH REACTIVE DOG, 2ND EDITION
Keep in mind if you observe similar reaction or behaviors in situation in which the dog is not on a leash the the treatment needs to take a different approach.