This happened on Thursday evening, 23:00 in the metro around the center area (between the stations of Palestro and San Babila). I am posting this as a warning to anyone who has the impression that it's enough to be careful.
I was in the metro M1 going home. It was fairly crowded given it was near the Duomo area. Six guys who seemed north-african with hoodies on and covering their faces just came and crowded me and my girlfriend who were sitting in the train. Then they suddenly jumped at me, starting pulling me and trying to grab me in order to steal my neckchain.
Once they successfully ripped it from my throat they ran off at Palestro; I went out and managed to take a photo of the group but most of their faces were covered.
I called 112, they were unable to do anything. First time I got in contact with an agent was only after around 15-20 minutes. They had plenty of time to run out of sight.
I went to the pronto socorso the next day so they could see the scratches in my neck and chest area and then I went to the police to file the report. This was on the 1st of May so it was done fairly quickly. But this also means the investigation will probably not start ASAP and I will probably not recover my stolen item.
I want to mention one more time the metro was quite crowded. You cannot rely on this. A lot of people were looking at it happening. As the assailants ran I could see the looks of terror on the people's faces but none of them acted at that moment.
Take care and hold on to or hide your valuables no matter where you are, everything happens in seconds.
PS: Not a tourist, been living here for a long time.
I decided to make this post knowing this happens to many people and also having read similar stories here many times.