Four foreign nationals have been arrested in the eastern Libyan city of Benghazi on suspicion of being Christian missionaries.
Preventative Security said they were under investigation for printing and distributing books about Christianity.
Proselytising is forbidden in Libya.
The BBC reports those arrested were an Egyptian, a South African, a South Korean and a Swede with joint US citizenship.
Preventative Security said diplomats had been allowed to visit them in detention, but would not say where they were being held.
Agents reportedly found the suspects in possession of 45,000 books about Christianity when they were arrested on Tuesday. Another 25,000 were thought to have been distributed.