He said that guards had hung him by his handcuffs for half an hour and pulled his underwear down, threatening him with rape
Belarusian writer Svetlana Alexievich, last year's Nobel Literature Prize winner, has signed a petition for the release of a Russian activist who says he has been tortured and threatened in prison.
Opposition activist Ildar Dadin, 34, alleged in a letter to his wife published Tuesday that he has been regularly beaten and threatened with rape and murder in prison, where he is serving a two-and-a-half year sentence for staging solo anti-government protests.
The letter prompted a wave of support from the human rights community and saw former Soviet-era political prisoner Lev Timofeyev launch a petition calling for the "immediate revision of his unprecedentedly unjust, unconstitutional sentence."
"I signed the petition because we are all waiting for some kind of change," Alexievich, who has often criticised authoritarianism in her native Belarus and neighbouring Russia, told AFP.
"Dadin was put in prison because he staged a solo protest. Let's all do like him (...) and do something worthy. It could change our lives."
Novelist Lyudmila Ulitskaya and a number of Soviet-era dissidents also signed the petition.
In his letter, Dadin said that on the second day of his detention in September, the prison governor and three other employees came to his cell and began beating him.
He also said that guards had hung him by his handcuffs for half an hour and pulled his underwear down, threatening him with rape if he refused to stop a hunger strike he had begun after being deprived of basic necessities such as soap and toilet paper.
Dadin was the first person to be imprisoned under a new law that punishes repeated participation in unsanctioned rallies with jail time.
Several international rights groups have also called for a thorough investigation into his allegations, as well as for his release.
"Dadin was convicted for nothing more than peacefully exercising his right to free expression, and that in turn has led to further serious physical abuse against him and even death threats," Yulia Gorbunova, a Russia researcher at Human Rights Watch, said in a statement Wednesday.
Regional authorities said Tuesday they would probe Dadin's claims, while Kremlin spokesman Dmitry Peskov said President Vladimir Putin would be informed of the situation.
International rights groups have decried inmates' treatment in Russian prisons, which are often overcrowded and rife with corruption.