Police hardly care. They'll arrive 15 minutes after someone has been beaten within an inch of their life, take a report, and do no investigating. CCTV that captured the whole thing? They won't even bother to collect it. That's too much work. Have seen this first hand. A good samaritan can make a real difference in the moment, with some large personal risk to themselves of course. The least you can do is make noise and make it clear that the police are coming.