The number of Muslim converts who are risking prison or death by secretly worshipping as Christians in Iran’s house church movement has grown to as many as 1 million people, according to watchdog groups.

The London-based Pars Theological Center is training at least 200 Iranian Christians to become the next generation of Iran's church leaders, the Christian Post reported.

The persecution of Christians has persisted in Iran since the 1979 rise of the country’s theocratic Shiite Muslim government -- with Christians facing the threat of death, lashing and torture. About 100 Christians currently remain imprisoned under Iranian President Hassan Rouhani's rule.

In 2010, Iranian Supreme Leader Ayatollah Ali Khamenei said the country's underground house churches “threaten the Islamic faith and deceive young Muslims.”

"If they want to sing, they have to sing very quietly or not sing at all."
— Observer of Iranian "house" churches

Despite the crackdown, there is now a growing movement of Christians in the Islamic Republic. Groups like Open Doors USA estimate around 450,000 practicing Christians in the country, while other estimates record more than 1 million Christians in Iran.

Christians in Iran are being driven to worship in secret house churches, and even there, they may face arrest. (Reuters)

"Pars sees this as a real chance to train agents of change who would transform the Iranian society from the bottom up by fostering a grassroots development of the values of Jesus in an Iranian style," a source close to the center told the Christian Post.

"It is not anti-Iranian," the source said. "It's an Iranian movement. It's a great, great number of Muslims turning to Christ."

The source also said that Iranian house churches consist only of about four to five members -- due to the threat of detection -- and that they are forced to their place of gathering every time they meet.

"If they want to sing, they have to sing very quietly or not sing at all," the source told the Post.

While Iran has released high-profile Christian pastors from captivity -- most notably Iranian American Saeed Abedini -- other Christian ministers still languish in the country's prisons.

Pastor Farshid Fathi has been locked up in Iran’s notorious Evin prison since December 2010 for what the American Center for Law and Justice describes as practicing his Christian faith.

After bowing to international pressure, Iranian authorities are starting to avoid charges that appear to be based on a person’s faith, according to the ACLJ. In Fathi’s case, his Christian activity was framed as being “criminal political offenses” by the court.

The regime in Iran equated his activities as “actions against national security,” based on evidence the pastor unlawfully distributed Bibles printed in Farsi, Iranians' language.

Another Christian minister, Pastor Behnam Irani, is serving six years in Ghezal Hezar prison for alleged “actions against the state,” after he preached to a group of converted Christians in a house church as well as sharing his faith with Muslims.

But the widespread persecution hasn't stopped groups like Pars from expanding the Christian movement within the Islamic Republic.

The center, which was founded by Rev. Mehrdad Fatehi in 2010, works closely with several Iranian house church networks. About 70 percent of Pars' students live in Iran and are trained within the country, according to the Christian Post.