The report, entitled "Isolation - Voices from the Basement," was drawn up at the request of conservative Euro deputy Anna Fotyga, a former Polish foreign minister.
It describes what it says are "human rights violations under international law committed by the Russians, and so-called separatists in the so-called Donetsk People's Republic."
During the armed conflict in the Donbas that has been underway since 2014, almost 4,000 Ukrainian citizens were taken prisoner by Russian-backed separatists, according to the report.
The document details harrowing accounts by former prisoners.
It says it brings together the testimonies of 16 prisoners held between 2016 and 2019 at the Donetsk Plant of Insulation Materials in eastern Ukraine, which is used as a pre-trial detention centre.
"The UN Human Rights Monitoring Mission in Ukraine documented cases when detainees were beaten, strangled, deprived of food, water, sleep or right to use the toilet, and subjected to electric shock, torture, mock executions and other forms of torture," the report says.
The report is available in English, Polish and Ukrainian.