For the "already done this":
https://en.wikipedia.org/wiki/COINTELPRO
For the modern technology making it significantly easier:
https://en.wikipedia.org/wiki/IMSI-catcher
https://en.wikipedia.org/wiki/Stingray_phone_tracker
TL:DR, if you're at a protest and your signal drops to 2G, your device is being monitored and tracked. Whether or not anyone acts on that data is a whole other thing.