[Mobuck]

If I were alone, I'd keep moving away if that was possible while keeping some sort of obstruction on at least one side(like parked cars , a fence, or building).
If actually approached or threatened, I would back against any available cover to prevent being hit from behind and draw my pistol while dialing 911.
If the threat continued, I would verbally warn the attacker(s) trying to make sure it would be heard on the 911 tape.
The next step is to fire at the closest/most dangerous threat. The pistol I carry holds 15 rounds plus at least one extra mag and I have no doubt I can stand my ground against a 180* threat area.
When the dummies start dropping, I am pretty sure the area will clear out quickly. I'm beginning to get the feeling that those who don't carry a firearm may just deserve to become a victim and I would have serious reservations about attempting to protect an able bodied male caught in this scenario. Children or older folks are another situation completely and I definitely would stick my neck out to help them.
To put this into perspective, I'm almost 60 and medium small statured so almost anyone bigger/younger than myself could be considered a serious threat of harm even with only a knife, club, or even fists.