You didn't say what Leica you have. They make a lot of compact cameras. On my last trip before the pandemic we went to London. As it was a vacation with my wife and not a photography trip, I took a compact camera which would fit in a front pocket of my pants. I do several things when I travel overseas: I use a day pack by Pacsafe, which has a metal mesh sewn into the fabric to prevent slashing, it has a steel wire in the shoulder straps to prevent slashing, and it has locks on the zippers. I also wear pants that have zipper pockets for the camera and hidden pockets for cash and credit cards.
If you have a larger camera, consider getting a strap from Pacsafe. They have steel cables inside to prevent cutting.
http://pacsafe.com/pages/anti-theft http://shop.bluffworks.com/products/ascender-chino-regular-fit-desert-khakiWith all that being said, I have never had anything stolen during travels to Asia, Central and South America, South Africa, and Europe. That is because I always stay aware of my environment, do not wander in unknown parts of a town or city, and avoid making myself a victim in other ways. Never look lost - always walk with purpose. Never stop for someone asking for money or the time. Never wearing flashy jewelry - I wear a $35 Timex watch which seems to tell time as well as a Rolex and does not invite theft.
Always assume in a crowd that you will be subject to pickpockets. Zippered pockets and inside-the pants pockets help. Never put valuables in your rear pocket. Always assume anyone bumping into you is trying to steal your stuff! In the Mercado Central in San Jose, Costa Rica, one of my fellow travelers was walking down the sidewalk. The young man in front of her turned around suddenly as if he forgot something, causing her to run into him. He apologized...but in the confusion his compatriot walking behind her had slit open her knapsack and helped himself to some of her stuff. But that trick happens everywhere - a college classmate in Boston was riding his bicycle back from school with his briefcase on the back rack. A young man stepped out in front of him and he ran into him, knocking everyone down. As he made sure the kid was OK, he realized that another kid had stepped out from between the parked cars and had taken his briefcase. He started to run after him, but quickly realized he wouldn't be able to catch him. Of course, when he turned around, he saw that the first kid was riding off on his bicycle!
The moral of those stories is to always be aware of distraction techniques. The only time I encountered a pickpocket was on an escalator in Paris at one of the Metro stations. A gentleman was walking up the escalator, and as he passed me (saying "excuse me" in English - he obviously had us pegged as tourists), he dropped his cigarette lighter on the stair in front of me. He leaned down to pick it up, and I realized that he was leaning against me and soon I would have to grab the rails with both hands or fall backwards into my wife, who was behind me. I immediately processed the thought: why is he doing this? OH CRAP - he is going to pick my pockets! We were nearing the top of the escalator - I shoved him hard and he went down. My wife looked at me as though I had gone crazy! He, on the other hand, immediately jumped up and ran away, never saying a word...
So - just be aware, be careful, but have a great time. Also - if you have not been to Paris before, we found the Rick Steve's Guidebook to be very informative. For example, get the 5-day (or whatever) Museum Pass. It allows you to skip the line to buy admission tickets, and some museums, like the Louvre, have a special entrance. You also then do not feel compelled to spend the whole day at the Louvre, since you can come back for a few hours the next day if you like... If you are unfamiliar with Rue Cler, read up on it. It is a photographer's dream.