I personally would never carry a backpack, just because you have no way of knowing what is going on behind you. A good pickpocket will unzip and dip into your backpack or slash the bottom without you even noticing it. A lock would certainly help a bit, as would not carrying your phones, credit cards and ID in it. I carry a small cross-body bag that sits in front of my body, that zips, and has a pocket where the zippered opening actually sits against my body, and I keep my credit cards & ID in that pocket. It's pretty hard to even get to. I am as vigilant as possible to make sure the compartments are zipped at all times. If I'm somewhere where I feel particularly vulnerable (big crowds, known for pick-pockets, etc) I have a pouch for ID & credit cards that goes around my neck under my shirt and bra. The phone I just try to shove deep into the main compartment, as I'm constantly using it now as my camera, and don't want to expose the credit cards & ID in order to pull the phone out.
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