You would have to keep your wits about yourself. Avoid bad & dark places, trust your gut. If you don't feel safe, don't go, back down and change direction.
I wouldn't recommend that you "fight" back. Many situations like that is unknown, even though you might be a 2nd dan Black belt, I always believe in running away quickly. You never know what kind of a weapon, the enemy might draw.
Besides this, you might also get into trouble with the local law. Bear in mind, you are in other people's country, so you would need to respect all their laws. I know that you might be in the process of being "robbed" but the rule of reasonable force can be very vague. And who the cops will trust.
If your group is "biggish", try to divide into groups of 3-4 members. Sort of like mini-platoon, and assign buddies so that they can keep watch on who is where. Plus keeping an extra eye on each other camera bags or back-packs.
2 years ago in Barcelona, while walking in the city, my wife & I was "stopped" by 2 fake "plain clothes policemen". This was wayyy before I started photography..... anyway, my smart wife asked to look at the ID again. And I literally stooped down to look at it.....
Lousiest fake ID, I have ever seen. I quickly put my arms around my wife, and whispered, "fake, fake, let's go now, quickly!!". Lucky thing, they did not give chase. Talk about almost "**** in the pants". :sweat:
If I had been blur, blur and handed over the passport, then finished..... GAME OVER.
As a rule of thumb, in most "developed" countries. No plain clothes policemen would "routine-stop" tourists for fear of mis-understanding. Generally speaking, only full uniformed cops with a big badge, name badge and all the insignia plus a 9mm side-arm is the real deal.
Enjoy your trip in Europe, although there are a few stories floating here & there. Generally, it is still an ok place. Keep your eyes & ears open, but have fun.