They are not the violent type - just those typical rather sharp-looking types who are watching out for any opportunity they can find to make a quick snatch - be it wallets, cameras, videos, or whatever.
Their presence seems to be increasing year by year and will do most of their work right in among the milling crowds. Although cars are obviously another favourite target.
Make sure you don´t carry too much money on you, nor any important documents. Make a fotocopy of your passport or identity card in case you have any problems. Leave everything you can at your hotel or whatever, or in the left-luggage office.
It´s just plain silly to take your camera round with you at nights, unless you are a professional photographer or something. It´s just going to hinder your enjoyment of the night, and maybe you could get it broken, nicked on you (especially if you are well - plastered) or you could just simply lose it.