Preventative Security, an intelligence division of the defense ministry, arrested a Swedish-American, an Egyptian, a South African and a South Korean, all of whom have been in contact with their respective embassies, Britain's Guardian newspaper reported Sunday.
Authorities said the foreigners were missionaries suspected of having a local printer create pamphlets explaining Christianity. A law held over from the Moammar Gadhafi era says the act of proselytizing is a criminal offense and could be punishable by death.
Libyan lawyer and human rights activist Bilal Bettamer said missionaries should not try to spread their faith in a wholly Muslim country.
"It is disrespectful. If we had Christianity we could have dialogue, but you can't just spread Christianity," he said. "The maximum penalty is the death penalty. It's a dangerous thing to do."
Libya has no known Christian minority, and the only Christian churches in the country serve foreign residents, The Guardian reported.