Oksana Shechko told the Associated Press that she and two fellow activists with Femen were interrogated on a bus for 12 hours, made to undress in the woods in freezing temperatures, punched in the head and back, threatened with death and rape, smeared with paint and had some of their hair cut off.
Shechko charged that the Belarusian authorities were punishing them for a protest in Minsk on Monday in which they bared their breasts to bring attention to President Aleksander Lukashenko's crackdown on the opposition.
"It was a real fight for survival," Shechko said after returning to Kyiv, her voice trembling. "We ended up in the hands of the butchers who kill and terrify the Belarusian people."
Shechko, 24, said that at one point the activists were made to sit still on a bus for several hours, prevented from speaking or moving.
"If we moved our heads or hands, they beat us, it was very painful," Shechko said.
The activists were finally left half-naked in a freezing forest, where they eventually found help from area residents and contacted the Ukrainian embassy.
Activist Inna Shevchenko's hair had been dyed green and some of it cut off in the back.
The three activists' protest took place on the steps of the KGB headquarters and was one of multiple demonstrations against Lukashenko on the first anniversary of his re-election. Agents quickly broke up that demonstration and arrested several journalists and Femen's Australian videographer Kitti Green, Femen said. The group said Green was deported to Lithuania.
Lukashenko has repressed opposition and independent media since becoming leader of the former Soviet republic in 1994.
In December 2010 elections, he was declared winner of a new term, but tens of thousands of protesters assembled to denounce alleged vote fraud. Police harshly broke up that demonstration and arrested around 700 people, some of whom remain in jail, including two of the candidates who opposed Lukashenko.
Femen activists have become popular in Ukraine for staging half-naked protests, and they have conducted some in other countries.