Assuming there is one attacker, I would intervene immediately, physically. I would pull him off the woman, and try to restrain him.
If he tried to run, I would try to get at least a piece of him, a license number, a description, something, that would help identify him. I would let him go, however, as my job at hand is not to catch a rapist, but to help a victim.
If he decided to attack me, I would protect my life with whatever force was necessary.
If they ended up being a kinky couple, (and I would like to think I would know the difference) then I would give them a strong piece of my mind for displaying thier perversions in my park, and call 911.
I have to say that I would call 911 post incident. I would not dwaddle on the telephone while someone was being brutalized in this fashion. The 10 seconds you spend talking to a dispatcher may be the victim's last ten seconds of life.
If there were more than one attacker.......... THEN I would dial 911 first, because I could concievably be a victim as well. I would then intervene verbally and try to get the attention focused on me. I would likely already have my weapon out, and would not hesitate to use it at the slightest provocation.
Right or wrong, I feel that this is how I would respond, unless someone effectively shows me the errors of my ways. I just cannot and could not get past the idea that that woman is some man's daughter, and hopefully if it were my daughter some man would be man enough to intervene.