Amnesty International says that it has gathered "graphic and compelling evidence of savage beatings and other torture" by pro-Russian separatist groups in eastern Ukraine.
The human rights group says that protesters and journalists have been targeted over the last three months.
An Amnesty report, Abductions and Torture in Eastern Ukraine, says that hundreds of people have been abducted.
Pro-government forces also committed a smaller number of abuses, Amnesty says.
Ukraine's military has made significant gains in recent days.
"The bulk of the abductions are being perpetrated by armed separatists, with the victims often subjected to stomach-turning beatings and torture. But there is also evidence of a smaller number of abuses by pro-Kiev forces," Amnesty's Deputy Europe and Central Asia Director Denis Krivosheev said.
"The time has come to take stock of what has happened, and stop this abhorrent practice."
Amnesty says that while there are no definitive figures on the number of abductions, the Ukrainian interior ministry has reported nearly 500 cases between April and June 2014.
It says that the UN Human Rights Monitoring Mission for Ukraine recorded 222 abduction cases over the last three months.
Amnesty International says that a fact-finding team has just returned from eastern Ukraine where they interviewed torture survivors on both sides of the conflict.
Amnesty report: Targeting of pro-Ukrainian activists
Hanna, a pro-Ukrainian activist, told Amnesty International how she was abducted by armed men in the eastern city of Donetsk on 27 May. She was held for six days before being released in a prisoner exchange.
"My face was smashed, he punched me in the face with his fist, he was trying to beat me everywhere, I was covering myself with my hands… I was huddled in the corner, curled up in a ball with my hands around my knees. He was angry that I was trying to protect myself. He went out and came back with a knife."
Hanna showed the Amnesty researchers scars on her neck, arms and legs where she said she had been "sliced" with a knife.
It says that the researchers were provided with a list of more than 100 civilians who have been held captive and that allegations of torture have been made in most cases.
"Abductions have taken place across eastern Ukraine, in the Donetsk and Luhansk regions," Amnesty says.
"Those targeted include not only police, the military and local officials, but also journalists, politicians, activists, members of electoral commissions and businesspeople."
Amnesty says now that pro-Kiev forces are re-establishing control over areas of the east "new captives are being released almost daily with an increasing number of disturbing cases emerging".
Amnesty report: Abuses by pro-Kiev forces
Pro-Kiev forces, including self-defence groups, have also been implicated in the ill-treatment of captives, Amnesty says.
One local government official in the south-eastern port of Mariupol told Amnesty researchers how they heard a captive separatist fighter "wailing in pain" at the hands of pro-Kiev forces who were "seemingly trying to extract information about the separatists".
While most abductions appear to have a "political" motivation, Amnesty says, "there is clear evidence that abduction and torture is being used by armed groups to exert fear and control over local populations".