Friendship bracelet or lucky ring
There are groups of scammers that pretend they are giving bracelets or rings for free. They usually will ask you, where are you from and say something about your country or say that they have a friend there. After that they will offer you a "free" bracelet and if you accept they will put it really tight in your arm. So will make it really hard for you alone to remove it. Then they will ask you money and if you deny they sometimes can become aggressive. If someone approaches you, keep your hands in your pockets and keep walking. Don't say a single word to them and ignore them is the best thing you can do.