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.
Russian rebel commander Strelkov ordered all men between 18-55 to fight or face a punishment yesterday, they didn't say how people who refuse would be punished.