The Great Purge of 1936-1938

At three publicized show trials held in Moscow between 1936 and 1938, dozens of these Old Bolsheviks, including Zinov'ev, Kamenev, and Bukharin, confessed to improbable crimes against the Soviet state and were executed.

(The last of Stalin's old enemies, Trotsky, who had supposedly masterminded the conspiracies against Stalin from abroad, was murdered in Mexico in 1940, presumably by the NKVD.) Coincident with the show trials against the original leadership of the party, unpublicized purges swept through the ranks of younger leaders in party, government, industrial management, and cultural affairs.

Death solves all problems - no man, no problem.”

— Joseph Stalin

According to the declassified Soviet archives, during 1937 and 1938, the NKVD detained 1,548,367 victims, of whom 681,692 were shot - an average of 1,000 executions a day (in comparison, from 1825 to 1910 the Tsarists executed 3,932 persons for political crimes). Historian Michael Ellman claims the best estimate of deaths brought about by Soviet repression during these two years is the range 950,000 to 1.2 million, which includes deaths in detention and those who died shortly after being released from the Gulag as a result of their treatment in it. He also states that this is the estimate which should be used by historians and teachers of Russian history.
According to Memorial society, on the cases investigated by the State Security Department of NKVD (GUGB NKVD):

At least 1,710,000 people were arrested
At least 1,440,000 people were sentenced
At least 724,000 were executed.

Among them:
At least 436,000 people were sentenced to death by NKVD troikas as part of the Kulak operation (see also figure of 376,202)
At least 247,000 people were sentenced to death by NKVD Dvoikas' and the Local Special Troykas as part of the Ethnic Operation
At least 41,000 people were sentenced to death by Military Courts
Among other cases in October 1936-November 1938:
At least 400,000 were sentenced to labor camps by Police Troikas as Socially Harmful Elements
At least 200,000 were exiled or deported by Administrative procedures
At least 2 million were sentenced by courts for common crimes, among them 800,000 were sentenced to Gulag camps.

Some experts believe the evidence released from the Soviet archives is understated, incomplete or unreliable. For example, Robert Conquest suggests that the probable figure for executions during the years of the Great Purge is not 681,692, but some two and a half times as high. He believes that the KGB was covering its tracks by falsifying the dates and causes of death of rehabilitated victims.