At least 254 Christians have been caught up in a wave of arrests, interrogations and detentions in the last year.
While most of those arrested were released following a brief incarceration, CSW says at least 41 were made to spend between one and eight months in prison.
Those imprisoned were subjected to unsanitary prison conditions and mistreatment, including solitary confinement and sleep deprivation. Medical treatment was also withheld from those who fell ill, CSW said.
Christians released from prison report the use of physical and psychological torture by interrogators as a way of pressuring Christians into making confessions or giving information about other Christians and church activities.
At least seven men are still in prison, including Pastor Yousef Nadarkhani, who is appealing his death sentence for apostasy.
CSW’s Advocacy Director Andrew Johnston said: “In light of the recent intensification of pressure on Christians in Iran, both CSW and Elam are united in the conviction that this is a crucial time for Christians in the UK to pray fervently for all Iranian Christians and particularly for those in detention.