If you have just been part of a crime then the advice from the Prefecture de Police English website is to,
If you need help, assistance or intervention, you can call the operators at the central Parisian police station or the inner suburbs department 24 hours a day.Prefecture de Police
In an emergency, dial 17 for the emergency police service.
If you only have a mobile telephone and you cannot, or the number 17 does not work on it, dial 112.
There seems to be a lot of crime going on in Paris at the moment. I have been here for four months and have been mugged already. Usually the criminals try and scam you in a non-violent but today I took a picture of a group of them who had been almost attacking a man. As soon as they saw me taking out my camera they covered their faces and started heading towards me. I don’t think they were to keen on being filmed.
One of them even looks to be holding up a picture in front of her face to defeat any facial recognition software.
This is a daily occurrence during tourist season. There is even a web page called 30 Tourist Targeted Scams in France where you can narrow down scams by city. There must be some sort of regional differences.
The important thing is to report the crime. Take photos or record if you can safely but report each one you see. Complain to the authorities. Make them take actions to keep tourists safe. Otherwise this is just going to get worse and worse and one day it will be you on the end of it.