Seriously at first take an evasive maneuver, then maybe attempt to apprehend the thrower.
Interestingly you can't in a lot of those cases. If you jump out from the line to snag someone in a protest/riot situation, you're likely to get dragged into the crowd and beaten senseless. It's sort of perverse, but a riot is exactly the one weird situation where people can throw a brick at a cop and not immediately feel the consequences... bit of a headscratcher.
Photo: thanks, based on the TG number and that, I think that's likely the one they were talking about.