Situational awareness is more than knowing where the bad guys are. You also need to know where the good guys are.
Before I go on I'll repeat that I think the scenario as stated is very unrealistic.
Unless a crowd is moving through a choke point or an enclosed area, there will be room to maneuver. Check out video from the WTO, various Occupy marches, and the recent May Day marches. You'll see that while the crowd is marching it is fairly easy to move about.
It's only once confrontation lines are formed between protesters and police that it becomes hard to move forward. Even then a few feet from the front lines it's still possible to move laterally.
So after the first confrontation, move to a place that gives you an avenue of retreat to one of the groups of good guys, either police or march organizers.
In the scenario given, a small minority of anarchists don't want to be photographed. This in the midst of many peaceful marchers that want very much to be photographed. That second group also wants to be seen as non-violent. So the odds of them helping out someone being threatened are very high.