Witnessed a man beating his wife/gf in public today, is there any way to report these things?

Was crossing the street in akiba today, at the end of a cross walk a guy slapped what I assume was his wife/gf as they were about the same age. I was kind of in shock then as I got closer I saw she was crying and he proceeded to kick then slap her. At that point I instinctually just went between them, got up in his face and yelled what the fuck man?? (I'm a woman btw, I thought maybe someone would also come back me up but nope) he just stared at me angrily then the woman waved me away and they sped walked away. I looked around for police or security of sort but couldn't find any.

My biggest surprise is that it was mid day, and there were plenty of other people both locals and tourists, just walking by as if nothing was happening. This is my first time in Japan, is this kind of thing common? I just kind of am in disbelief. Is there any protocol to go about reporting this if I see it again?