The armed Iranian opposition group, the Peoples' Mujahideen, has been accused of mistreating its own members by an American human rights group.
The groups political wing says it is a democratic alternative for
Former members of the group, also known as Mujahideen-e Khalq, told Human Rights Watch of abuses of dissident members and of those wanting to leave.
They included detention, solitary confinement, beatings and torture.
For more than 15 years before the fall of Saddam Hussein the group used bases in Iraq to launch attacks against Iran.
The report says that the release of a group of dissident Peoples' Mujahideen members from Iraq's Abu Ghraib prison during Saddam Hussein's last year in power provided, for the first time, a direct window into conditions inside the group's camps.
The report was based on interviews with some of those released and other former group members living in Europe.
Marxism and Islam
BBC regional analyst Pam O'Toole says the Peoples' Mujahideen is an authoritarian organisation whose ideology combines elements of both Marxism and Islam.
Several interviewees who shared a prison cell in the mid-1990s told Human Rights Watch that a cellmate died after a severe beating.
One man described being held in solitary confinement for eight-and-a-half years, another for five years. Both were high ranking members who wanted to leave.
Eventually they were handed over to Saddam Hussein's Iraqi authorities who held them in Abu Ghraib jail and later repatriated them to Iran.
The organisation is designated as a terrorist organisation in the US and Europe.
But Human Rights Watch says its political wing - the National Council of Resistance of Iran - continues to lobby Western legislators to get that designation lifted and is presenting itself as a democratic alternative to the Iranian government.
Human Rights Watch says while Iran has a dreadful human rights record, it would be a huge mistake to promote an opposition group that was responsible for serious human rights abuses.
After the fall of Saddam Hussein the US military disarmed the Peoples' Mujahideen fighters inside Iraq and confined them inside the group's main base.