But to no avail.
Physically detaining them is very difficult, as they're trained police special forces.
Were one to be physically held long enough for police to arrive, police would surely allow the rock thrower to flee, and arrest the civilians for "unlawful arrest."
That's what happened during the protests in September 2017 (over the killing of Santiago Maldonado) and the attack on the Ateneo Theater last August.
It's very difficult to find a judge who'll allow any case against police provocateurs to go forward, moreover, given what a number of judges have described as unprecedented pressure from Macri (including espionage and extortion).

Déjà vu: This police woman has been among the several officers identified as provocateurs; she can be seen attacking the Ateneo Theater in August 2018, and in the Maldonado protest incidents in September 2017.

Police Sargeant Héctor Olivera (posing as a rock thrower) has a wardrobe malfunction during the September 2017 incidents. He was said to be in yesterday's riot as well, though that hasn't been confirmed.
Here are some of the police/state intelligence provocateurs they've identified thus far; the courts won't touch them:

Thanks as always for taking the time to read, and share your thoughts and questions, Judi. One can only hope that under the next administration, all this might be investigated.
As many serious other problems as they'll inherit from Macri though, I wouldn't blame them entirely if they decided to turn the page on this one.