A 78-year-old woman is among the Christians in Iran who have been arrested by the authorities in recent weeks.
According to Barnabas Fund, the Iranian authorities have initiated a wave of arrests across the country, targeting ordinary church members and leaders.
Security agencies have swept house churches in Ahwaz, Shiraz, Esfahan, Tehran and Kermanshah since Christmas.
The arrests have taken place inside homes and workplaces, and even some members of the officially registered churches have been seized.
Churchgoer Giti Hakimpour, 78, was arrested in a raid on her home in Esfahan at 6am on 22 February. Officers searched her apartment and confiscated some of her belongings. She was released three days later.
Hekmat Salimi, the pastor of a church in Esfahan, was arrested on the same day as Hakimpour. Security agents ransacked his home and seized his computer, books and other personal belongings.
In some swoops, entire congregations have reportedly been rounded up. In Kermanshah, authorities entered a house church and arrested all 13 Christians present. Three are still in custody.
Barnabas Fund said the international pressure on Iran from the West over its nuclear programme could make the Iranian authorities intensify their campaign against Christians, who are regarded as allies of foreign powers because of the misconception that Christianity is a Western religion.
Despite the wave of arrests, Barnabas Fund reports that the church in Iran is experiencing rapid growth.
An Iranian Christian told the organisation: "I feel sorry for the Iranian authorities because they dont realise that theyre up against the Holy Spirit."
Dr Patrick Sookhdeo, International Director of Barnabas Fund, said: "The authorities are turning the screw on our brothers and sisters in Iran and it appears that none of them, not even elderly church members, are safe from this campaign of intimidation and harassment.
"Those who are detained, and their families, often need practical assistance.
"As pressure on Iranian Christians intensifies, this need is growing, so I urge our supporters to give generously to this sensitive ministry."
Barnabas Fund also expressed concern for Masoud Delijani, a Christian convert from Islam, who has been jailed for three years. The charges relate to being a Christian, holding illegal house church gatherings, evangelising Muslims and action against national security.
According to the organisation, Masoud, a school teacher in Kermanshah, was held in solitary confinement following his arrest in March last year and was not given the opportunity to defend himself in court.