The UN's Ukraine-based monitoring team based its findings on interviews with more than 100 prisoners of war on each side of the conflict.
The interviews with Ukrainian prisoners of war were conducted after their release, since Russia did not grant access to detention sites, it said.
Matilda Bogner, head of the monitoring mission, told a Geneva press briefing that the "vast majority" of Ukrainian prisoners they interviewed held by Russian forces reported torture and ill-treatment.
She gave examples of dog attacks, electric shocks with Tasers and military phones and sexual violence.
She said the treatment was aimed at intimidating and humiliating them. One man in a penal colony near Olenivka told the team that members of Russian-affiliated armed groups "attached wires to my genitalia and nose and shocked me. They simply had fun and were not interested in my replies to their questions."
Russia, which invaded Ukraine on February 24, denies torture or other forms of maltreatment of POWs.
On the Ukrainian side, Bogner reported "credible allegations" of summary executions of Russian prisoners among other abuses.
Other Russian prisoners reported poor and humiliating conditions of transport and of being packed into trucks or vans naked, with their hands tied behind their backs. The UN team said it had also documented cases of so-called "welcome beatings" at a penal colony.
Kyiv has previously said it checks all information regarding the treatment of POWs and will investigate any violations and take appropriate legal action.
