When did the campaign of political  repression that Mikhail Demichev was arrested during occur?
Great Purge The Great Purge or the Great Terror (Russian: Большо́й терро́р ) was a campaign of political repression in the Soviet Union which occurred from 1936 to 1938. It involved a large-scale purge of the Communist Party and government officials, repression of peasants and the Red Army leadership, widespread police surveillance, suspicion of "saboteurs", "counter-revolutionaries", imprisonment, and arbitrary executions. Mobile gas vans were invented to execute people without trial. In Russian historiography, the period of the most intense purge, 1937–1938, is called Yezhovshchina (Russian: Ежовщина ; literally, "Yezhov phenomenon", commonly translated as "times of Yezhov" or "doings of Yezhov"), after Nikolai Yezhov, the head of the Soviet secret police, the NKVD, who was later killed in the purge. It has been estimated that 600,000 people died at the hands of the Soviet government during the Purge.
1968 Red Square demonstration The 1968 Red Square demonstration (Russian: Демонстра́ция 25 а́вгуста 1968 го́да ) took place on 25 August 1968 at Red Square, Moscow, Soviet Union, to protest the invasion of Czechoslovakia by the Soviet Union and its Warsaw Pact allies, that occurred during the night of 20–21 August 1968, crushing the Prague spring, a set of de-centralization reforms promoted by Alexander Dubček.