Four foreign nationals have been arrested in the eastern Libyan city of Benghazi on suspicion of being Christian missionaries, officials say.
A spokesman for Preventative Security said they were under investigation for printing and distributing tens of thousands of books about Christianity.
Proselytising was forbidden in the predominantly Muslim country, he added.
Those arrested were an Egyptian, a South African, a South Korean and a Swede with joint US citizenship.
The Preventative Security spokesman said diplomats had been allowed to visit them in detention, but would not say where they were being held.
"We are still holding interrogations and will hand them over to the Libyan intelligence authorities in a couple of days," Hussein bin Hamid told the Reuters news agency.
Agents reportedly found the suspects in possession of 45,000 books about Christianity when they were arrested at a publishing house on Tuesday. Another 25,000 were thought to have been distributed.
Last year, the International Committee of the Red Cross (ICRC) had to suspend its activities in eastern and central Libya after its offices in Benghazi and Misrata were attacked.
The aid group was accused by some people of proselytising activities and distributing Bibles to internally displaced Tawargha people in Benghazi - accusations it strenuously denied.
Preventative Security was set up by rebel commanders during the conflict which ousted the late Libyan leader Muammar Gaddafi in 2011.