First off, you arse is probably fatter than mine, so no worries there...
As far as yelling obscenities, Honestly I don't know, as noted, never protested before. Is there a differencec between hollaring obsenities and telling a cop he has a fat arse?
That said, is it agasint the law to call cops name while your protesting? Honestly don't know. If it's agasin't the law, I guess the police could arrest the people, correct?
Generally speaking, I'd guess if you're doing riot duty as a LEO, is it all right to slug someone for calling you name? I don't believe so. Just like it wouldn't be ok to let the dogs loose on people for the same reason.
Keep in mind, just becuase you're right (or within the law) doesn't always mean you should do it IMO.