Amid a growing crackdown on religious freedom, Iranian police reportedly have been rounding up people they suspect have converted to Christianity.

On May 11, police arrested eight people in the southern city of Shiraz, according to Carl Moeller, president of Open Doors USA, a Christian organization that fights religious persecution.

Converting from Islam is a crime in Iran; converts can face jail and other penalties.

Most of those detained have been released, but at least one of them, 21-year-old Mojtaba Hussein, is still behind bars and is not cooperating with his captors, according to Moeller.

“He may not be willing to give up the names of other Muslim converts. He may not be willing to recant his faith himself,” Moeller said.

Numerous calls to Iranian government representatives in the U.S. have not been returned.

Though they are protected under the Iranian constitution, Christians are not given the same freedoms as other citizens in Iran. Christians can’t worship freely or hold public office, and they can be arrested for even speaking to Muslims about Christianity.

“Such people are persecuted, and particularly in the 1990s such converts were killed — it’s thought by government agents,” said Paul Marshall, a senior fellow at the Center for Religious Freedom at the Hudson Institute, a conservative think tank.

Under the watchful eye of the Iranian government, many have been forced to worship in secret and are moving underground into what are called house churches —although some sects, such as Armenians and Syrians, have been allowed to worship in churches.

“With [Mahmoud] Ahmadinejad as president, the Iranians are intensifying the religious dimension of their rule,” Marshall said. “The concern about the religious purity of the regime has become stronger in the last two or three years.”

Earlier this year, Ahmadinejad proposed a law that would impose a death sentence for any Muslim who converts to another religion. Under current laws, those charged with converting can be prosecuted and face jail time for vague crimes like “blasphemy” and “insulting Islam.”

Marshall said these restrictive policies may be creating a backlash among Muslims. “There are indications that with the deep unpopularity of the regime that people are turning away from Islam,” he said.

“Seeing Muslims converting to Christianity is directly threatening to an Islamic regime,” said Moeller.

He compared these small groups of converts to early Christians living under the yoke of the Roman Empire, who met in secret and whose beliefs were “dependent on dreams, visions, signs and wonders.”

Because Bibles are rare in Iran and teachings are not "as dependent on the Bible as Evangelical Christianity in America is,” said Moeller, there is a “real lack of scriptural foundation."

But despite the growing pressure from the state, worshippers continue to practice, and Moeller said the house church system seems to be growing.

“We’ve got confirmed reports of groups of Muslim convert believers doubling in size in the last six months,” he said.