If demonstrations were scheduled for something the Stalinist's opposed or against something they wanted, agents were sent to the site to infiltrate the ranks, inflame or perform violent, provocative acts so that the government would have an excuse to round them all up, imprison and try the demonstrators. This became so effective for the NKVD (KGB) they relied on it as SOP.
The purpose was not only to imprison/kill opposition members, but also to portray them and their cause as illegitimate for propaganda. Stalin often characterized those who opposed him as not only "counter-revolutionary" but eventually as criminals, and insane. That way the government was never required to deal substantively with any of the issues the opposition raised - they were insane or criminals and therefore anything they believed or said was to be disregarded.