The PCs saw the mob when they exited a public transit station in a bad neighbourhood. There were 40-50 thugs, most wearing green armbands and either carrying signs, clubs or guns and shouting anti-Vargr slogans (none of the PCs was a Vargr, but they were offworlders which was nearly as bad).
The four PCs wisely tried to cross to the other side of the road and move along minding their own business ... so I had five thugs decide they should try picking on them.
When fighting broke out the PCs wisely avoided using deadly force, but the badguys didn't. It was a very high law level world, so I had the rest of the mob move along once the first gunshots went off.
It worked out well, but I did cheat and only confront the PCs with 5 thugs. Realistically this is probably OK as you rarely get more than that many guys willing to do the same thing in any one mob/riot anyway.
Now, I've never considered running a riot (though now that I think of it...) but I know how to start one! Drop Trader Jim anywhere on planet and a riot will follow!