If the roundup of peaceful protesters is the result of a State of Emergency as invoked by Prayut for Bangkok, the action is illegal according to the Thai courts.
In February 2014 the Court ruled against PM Yingluck using emergency powers under a 60-day State of Emergency to disperse peaceful protesters. See asia.nikkei.com and Reuters.com.
Ancillary to the ruling (as I recall) was that the police use of water cannons and tear gas to disperse peaceful protesters in December 2013 was illegal even as protesters forcibly entered and occupied Government House. Effectively, the Cour