Missionary groups say the number of Moroccan Christians has grown to 1,500 from 100 in a decade and that Algerian Christians number several thousand, although no official figures exist.
They say their message is reaching thousands more, thanks partly to satellite TV and the internet.
The Koran states no-one can be forced to follow one religion, but many Muslims believe that to abandon Islam is to shun family, tribe and nation and bring shame upon relatives.
"Many Muslims told me 'If I find you I will kill you'," said Amin, a young man from northern Morocco who did not want to give his full name for fear of reprisals.
Amin said he became aware of Jesus Christ after dreaming that a figure dressed in a white robe approached him in a forest and handed him a Bible.
"When I told my father I had become a Christian he just stared at me without speaking. Then he said: 'From now on, you are not my son. Go to those people, let them feed you and give you a home -- we'll see who cares for you'," said Amin.
He left town, stopped his studies and now lives from translation work offered by a Christian missionary group.
Mission groups in North Africa range from broad alliances such as Partners International and Cooperative Baptist Fellowship to small Baptist and Pentecostal churches based in the Americas and Europe, according to their Web sites.
Their activity is growing as churches turn their focus to places where the Christian message is rarely heard, said Dana Robert, world Christianity professor at Boston University.
"With the internet and the increase in travel, you have a democratization of missions where anyone who feels like it can go anywhere they want," said Robert. "The new breed of missionary doesn't have the same historical training as the older established denominations, nor necessarily the cultural training, so there's a bull-in-a-china-shop effect."
Converts recount stories of persecution as evidence of the risks they run. These are impossible to verify, but one said he heard a newly converted Moroccan was thrown from a balcony in a shopping mall by two acquaintances, leaving him paralyzed.
Another said people of a town in eastern Morocco threatened to decapitate a convert unless he renounced his faith.
Islamic leaders say missionaries exploit people with a weak understanding of their religion, target the poor and the sick and try to win over north Africa's Berbers by telling them Islam was imposed on them by Arabs. Continued...