A Yemeni security official said police were cracking down on Muslims who have converted to Christianity.
The official said at least nine people have been detained in recent months. The official spoke on condition of anonymity because he was not authorized to speak to the media.
Rights activists and a family member of one of those detained said they feared those arrested could be tortured or abused in prison.
Conversion from Islam to any other religion is illegal in Yemen.
Separately, the official said police also have arrested six Iranians in Yemen who are followers of the Baha'i faith for allegedly belonging to a rebel group. Though not officially banned in Yemen, the Baha'i religion is considered by some Muslims as heresy.