Did the Soviets commit mass incarcerations and large scale human rights violations like the Nazis in Germany? I want to know this because I recently learned about the Victims to Communism monument in Canada and also I want to know if:

1. The information about gulags and camps in Soviet Russia is substantiated by some evidence.

2. Or is it some sort of campaign by detractors.

3. If the atrocities were committed, who were the most  targeted?

I am asking all these questions out of pure curiosity.

A response addressing all or some of them would be appreciated, as well as essential readings, if any.