Providing you dont have a smartphone, or more accurately something build more like a k750i, held in your fist with your thumb over the top it makes a good bludgen. Any set of keys with a single key braced between your 2 middle finger and supported from the back so as not to cut yourself also makes a good makeshift knuckle duster. A solid metal ring adds to any punch, though it will injure your finger and damage the ring, your call.
Be careful doing this. If you start pulling weapons no matter how makeshift and THEY have weapons they thus far haven't used, you might have just escalated the whole issue from a beating to actually defending your life.
Remove any items like watches or glasses the moment you think there might be trouble, as well as tying up your hair if needed.
If your attacked with weapons a jumper or a belt make good defences if you can get them off without opening yourself up, but you must be aggressive once you have neutralised the weapon or youll lose your defence and they will get their weapon back.
Generally speaking, the best tactic in these instances it to try and drop the first person who comes at you as fast and brutal as possible. Even most black belt would have a hard time going 7 on 1 if they all rush together, fortunatly these groups are usually lead by one or two individuals with the rest just following, you you can drop these first 1 or 2 before the other move forward, and do it in a way that they wont be coming back (IE a bloody nose isnt going to work, you need to snap and arm or KO them or alike), then you break moral in a horrific way and generally can buy yourself enough time to escape providing you don't show weakness.
Sometimes it'll go tits up though, and you'll still get the shit beaten out of you unless you actually kill someone, which sadly is the only thing not covered under self-defence as "reasonable force"
Tbh as well, its sad to say. But if you dont think youll be able to drop one of them, and you dont think youll be able to get away. Its often just better to accept what your guna get. Fighting back, regardless of what anyone else tells you escalates these issues if you dont do it right first time. Second time round they will be more careful and more brutal, to insure you dont get a look in. If you dont even run, they will become bored much faster, because they haven't had to work to get you, part of kicking the crap out of someone like that is running em down first. Its base, animal nature. They arnt after killing you, they want sport. Its no fun if you dont run.