We have compiled a small list of things to keep in mind when things go south:
Nobody expects you to handle everything by yourself.
Even more so, it is advisable to not even try that (we will get to that in a minute).
The first thing you always should do is to call for help. First of all, that means calling the police or an ambulance or other officials. They are trained for emergencies.
Next, inform other people around you. The easiest way to do that is to use enCourage to inform every around you.
Not acting doesn't mean ignoring. Contrary to popular believe, a silent bystander can be quite helpful.
Calling for help, assembling more people, observing details and jotting down important information.
Just by being present, you don't draw as much attention to yourself and can focus on other tasks.
Also, don't underestimate the moral support you can give to the victim by just being near.
Additionally, acting could also have consequences for yourself:
When presented with an offender, physical intervention is a priviledge that is reserved to the authorities, so by actively intervening you might break a law yourself. Even wrongly administered help might make matters worse, no matter your intentions, so when in doubt it is better to overthink your actions a second time.