There are many techniques that have been developed by DOD and DHS for crowd control. These include intense sound waves, light and sound effects that cause disorientation and nausea, and a range of imaging/surveillance technologies.
This includes the ability to track the trajectory of projectiles back to their source and target illumination (high-intensity spot lights?) and video resources to that location. Drones, helicopters and or strategically placed tracking cameras can focus on the location and track multiple individuals to points of intercept. Water drops and water cannons are used around the world. There are safe, but debilitating chemicals as well.
High-resolution, high-dynamic range IR imaging and other remote sensing technology can look through masks, umbrellas, clothing, etc. to detect weapons and identify individuals.
This technology all exists.