Amnesty says it has been told by civilians they had endured "abusive screening processes" – known as filtration – which sometimes resulted in arbitrary detention, torture and other ill-treatment.
Russia probably committed crimes against humanity by forcibly transferring Ukrainian civilians in Russian-controlled areas of the country to other regions, Amnesty International gas said.
It said on Thursday that civilians were moved from Ukraine further into Russian-controlled areas or into Russia, with children separated from their families in violation of international humanitarian law.
Amnesty said it had been told by civilians they had endured "abusive screening processes" – known as filtration – which sometimes resulted in arbitrary detention, torture and other ill-treatment.
"Separating children from their families and forcing people hundreds of kilometres from their homes are further proof of the severe suffering Russia's invasion has inflicted on Ukraine's civilians," said Agnes Callamard, Amnesty International's secretary general.
"Russia's deplorable tactic of forcible transfer and deportation is a war crime. Amnesty International believes this must be investigated as a crime against humanity," she said.
Amnesty said that in one case a woman was separated from her 11-year-old son during filtration, detained, and not reunited with him.
'Distress and anger'
The rights group said it interviewed 88 people, the majority civilians from Mariupol, the Ukrainian Black Sea city seized by Russia, as well as residents from the Kharkiv, Luhansk, Kherson and Zaporizhzhia regions.
"Most, especially those from Mariupol, described coercive conditions that meant that they had no meaningful choice but to go to Russia or other Russian-occupied areas," it said.
The transfers "amounted to war crimes and likely crimes against humanity," it said.
The report is one of the most significant interventions by Amnesty over Russia's incursion of Ukraine since it angered Kiev in August by publishing a report that accused Ukraine of endangering civilians by establishing bases in schools and hospitals.
The group at the time said it stood by the findings of the report, which prompted the head of Amnesty's Ukraine office to resign in protest, accusing the rights organisation of parroting Kremlin propaganda.
But it also acknowledged the "distress and anger" that the report had generated.