"Police in Kiev face serious challenges but nothing can justify the vicious beatings we've documented," Yulia Gorbunova, a Ukraine researcher for the rights group, said in a statement from Kiev.
"These are very serious crimes that need thorough investigation and the people responsible should be held accountable."
Protests erupted in Ukraine in November when President Viktor Yanukovych backed away from trade agreements with the European Union to protect economic ties to Russia.
Protests turned deadly last week after Yanukovych backed legislation restricting the right to protest.
Gorbunova said riot police tortured a 17-year-old protester who was arrested during recent skirmishes.
"There is no other way to describe it," she said.
The European Union said Neighborhood Policy Commissioner Stefan Fule was headed for Ukraine and that the country may face consequences if it doesn't stabilize the situation.