Torture methods included the use of electric shocks in interrogations, and the “principal targets” were suspected Ukrainian informants, a commission of UN investigators said.
The alleged behaviour of Russian troops could amount to crimes against humanity, with some victims having died as a result of being tortured, the inspectors said.
The focus of the panel's latest inquiries is Russian-occupied territory in Ukraine's southern Kherson and Zaporizhzhia regions. The torture took place in detention centres controlled by Moscow's troops, the commission said.
Torture was used “during interrogation sessions, mainly aimed at extracting information from the victims”, said Norwegian judge Erik Mose, the chairman of the three-member commission.
“This led to severe pain and suffering.”
One alleged torture victim was quoted as saying: “Every time I answered that I didn’t know or didn’t remember something, they gave me electric shocks.
“I don’t know how long it lasted. It felt like an eternity.”
Mr Mose, whose fellow inspectors are from Colombia and India, said torture appeared to have been “widely used” by Russian forces against those suspected of being informants.
“In some cases, torture was inflicted with such brutality that it caused the death of the victim,” he said.
Investigators have also raised the alarm about alleged sexual violence against women in Kherson and reports of children being forcibly relocated to Russia.
The commission said it had received no answers from Russia to its allegations. Moscow has routinely denied claims of war crimes and described them as Ukrainian provocations.
There have been a “few cases” of violations by Ukrainian forces, Mr Mose said, which he called on Kyiv to investigate.
Prosecutors at the International Criminal Court in The Hague issued an arrest warrant in March for Russian President Vladimir Putin over the alleged deportation of children.
Moscow and Kyiv resumed a legal tussle at The Hague's other world court, the International Court of Justice, on Monday over the Kremlin's claims it invaded to stop the “genocide” of Russian speakers. Ukraine wants judges to declare this a spurious pretext.