It is at this point trivially easy to locate strong signals in crowds - iirc the Olympics was one of the pioneers of the technology to prevent illegal broadcasts, and the equipment is fairly cheap and very easily accessible for any interested party that wants it. Foxhunter fourms even have many, many plans to build your own versions of the hardware if you want.

And that’s setting aside that the strength of a portable jammer is very limited, military radios like the US police use have pretty neat anti-jamming features (hardened FHSS is brutal, just look at Ukraine/russian signal warfare for how hard it is to mitigate) and you’d be absolutely screwing over any coordination attempts by other protesters using cell or radio by trying this.

You’ll get caught, and you’ll do a great deal more harm than good if you use these in a protest. Outside that context they could be useful, but by turning one on you gotta remember you’re ensuring that you can’t get any help, either.