It is a real threat in urban areas these days and a serious predicament if you happen to become a victim of such an assault. Avoiding places where youth gangs hang out is good advice. I've been in that situation at least twice. Once I managed to get away without getting physical. Once I got blindsided by the biggest guy while a couple of others distracted me and caught a sucker punch but since I didn't go down (I used to box a bit) the guys freaked out, backed away and a passerby defused the whole thing before it turned into a big fight.

In my experience, confrontations with youth gangs always start at a verbal level and often stay there. It's mostly about ego. In any gang most guys are basically cowards and don't want to fight (though they will happily assist in beating a helpless victim if they can without risk to themselves). But there is always at least one leader/tough guy who looks for trouble in order to impress the group. The usual tactic is to surround a victim and intimidate him. If you hand out your wallet or cellphone at this time it may not help. Maybe they're looking for money, maybe it's about something else. If they smell fear they will likely press you even harder, very simple predator mentality.

Good verbal skills and taking a confident but not aggressive stance seems to work best. If you can run away, good. But if you have to fight you must understand that you're heavily outnumbered. If you let them surround you you won't last more than a few seconds. Though most groups don't expect the victim to fight back. If you're the one to initiate the fight and manage to disable at least the leader there will be serious panic among the rest of the group. The advice from people who I know could actually handle themselves in that kind of fight is always go for the leader, strike first, keep all the assailants in front of you, maybe incapacitate the closest/weakest attacker and use him as a shield. Any kind of weapon is a big asset but it has to be deployed instantly before anyone knows what's going on and once you do it there's no turning back.

It is possible to fight your way out through multiple attackers but only if you're totally comitted and have the skills and attributes to pull it off. Look into Krav Maga or military combatives (WWII), they deal with this. If unarmed, strike hard to vital areas and move as fast as you can. The idea is not to incapacitate every guy in the group Bruce Lee style but rather to stun and disorient the leader and any guys directly blocking your way so you can escape as quickly as possible. A concealed blade can be extremely effective, aimed at the face or hands. But once you deploy a knife it's kill or get killed. In a civilian enviroment that probably means going to jail even if you survive the fight. Nothing to be taken lightly and a whole lot easier to talk about it than do it for real.