This includes the best available numbers on the people imprisoned and executed for counter-revolutionary (political, kulaks, etc.) crimes from 1921 to 1954 in the entire Soviet Union (includes republics annexed in 1939) from a report ordered by Khruschev (ie - the real numbers, not speculation), as well as an analysis of the commonly published speculative numbers:
642,980 executed, 2,369,220 imprisoned, 765,180 internally exiled over a 30-year period. That is 21,433 people receiving capital punishment each year, and 78,974 imprisoned per year. To put it in perspective, 89,000 people were sentenced (up to a maximum term of 5 years) for cannabis possession in the UK in 1998.
If you include the number of excess deaths from the Ukrainian and Kazakh famines of 1932, the direct and excess deaths from the civil war and the civil war famines, there's still no comparison to the Third Reich.
According to Solzhenitsyn a quarter of the population of Leningrad were arrested in and around 1937. Apparently this turned out to be a conservative estimate.
edit: I'll post some sources when I get home