How well protected were human rights in the USSR?
Human rights, from the Western/democratic perspective, were essentially non-existent in the Soviet Union.
In the Soviet Union, rights were officially guaranteed by the constitution. However, this was not really true in practice. People did not have any of the rights that Westerners take for granted. They did not, for example, have the freedom to criticize the government either in speech or in writing. They did not have the freedom to assemble or to petition the government. In all of these cases, the government was willing to jail anyone who spoke out. They did not have freedom of religion because the government was officially atheist and saw organized churches as a threat to their power. They did not have the freedom to move around in their country or to leave the country.
The USSR was a totalitarian system with no limitations on its power. It was quite willing to deny its people their basic human rights whenever it felt those rights were in conflict with its own ability to stay in power.