This is not constitutional. Even I know that. 

Federal agents arrest Portland protesters in unmarked cars, sparking intense backlash – The Washington Post:

“Arrests require probable cause that a federal crime had been committed, that is, specific information indicating that the person likely committed a federal offense, or a fair probability that the person committed a federal offense,” Orin Kerr, a professor at University of California at Berkeley Law School, told The Post. “If the agents are grabbing people because they may have been involved in protests, that’s not probable cause.”

There’s something to be said for police being easily and readily identifiable, too. Law enforcement officer identification must be transparent and easily recognizable. Otherwise you’re just some dude with a gun.