Yesterday, the alien wizard Handwavius flew by our planet. He saw how violent we were and decided to put a stop to it. He cast a spell on all humans to make us stop us being violent, then flew away again, thinking his mission was done. Now we have to live with the consequences.
From now on whenever a human decides to commit a violent act, they can't bring themselves to do it. They can imagine doing the act just fine, but not actually perform it. This is a mental block, not a muscle one. It will also apply to new humans.
Violence here is defined as "causing physical harm to other humans", and however the individual human interprets that. If you think, on some level, that your action will harm someone - then you can't do it. This doesn't apply to psychological harm, harsh words work just fine. Neither are you compelled to act to save someone in danger, you can only be forced to nonaction.
The block also apply to chains of actions. You can build a robot with a gun, but you can't program it to shoot people. Neither can you order someone to program a robot to shoots people, and so on - if violence is the intended end result, the action is prohibited.
The only way to cause violence is to literally not understand that your acts could cause harm to someone. Violence can therefore still be caused by young children, some severely mentally handicapped people, and indirectly by people not realizing the consequences of their actions. Nonhumans aren't covered, including animals and eventual AIs.
How will law enforcement work when things settle down a bit? Can you stop or punish those committing certain crimes, and if so, how? Will new crimes or punishments be added to the law books to compensate, or old ones removed? Assume most current governments manage to stay in power.
Edit to clarify some things: You can be compelled to nonaction, but not to action. This means that if you're unsure, or you think any action will lead to harm, you will do nothing. If you think an action will do small harm and inaction will do greater harm you will do nothing. Also, a psychopath that has no empathy will still not be able to hurt people. You would have to be damaged enough to not understand what violence even really is to be able to cause it.
This is an attempt to narrow down a too-broad question that I posted earlier and then deleted. If popular, I might have follow-up questions about other aspects later.