Ethiopia was one of the first Christian countries in the world.
Some 35 Ethiopian Christians face deportation from Saudi Arabia for "illicit mingling", the global rights body Human Rights Watch (HRW) says.
Police arrested the group - including 29 women - after raiding a prayer meeting in the second city of Jeddah.
The women were subjected to strip searches and the men beaten and called "unbelievers", according to HRW.
In 2006, the Saudi government promised to stop interfering with private worship by non-Muslims.
The group was arrested in a private home as they gathered to pray during the run-up to Christmas, celebrated by Ethiopian Orthodox Christians on 7 January.
HRW spoke to a man and two women by telephone from the prisons where they are being held.
They say they have been charged with mixing with unmarried persons of the opposite sex - even though HRW says Saudi Arabia has no law defining "illicit mingling".
Mixing of the sexes is not allowed in public - but normally permitted in private unless for "the purpose of corruption", according to the religious police.
The ultra-conservative Muslim kingdom bans the practice of any religion except Islam - but in recent years pledged to leave people of other faiths alone if they worshipped in private homes.
Ethiopia was one of the first Christian countries in the world, having officially adopted Christianity as the state religion in the 4th Century.