The Religious Liberty Commission of the World Evangelical Alliance reports that 53 Ethiopian Christians were arrested by Saudi authorities earlier in the month.
The arrests happened during a worship service in the privately rented home of an Ethiopian believer in Dammam, the capital of the Eastern Province of Saudi Arabia.
In total, 46 women and six men were arrested. The group included three church leaders.
The RLC said it was likely that those without residential permits would be deported.
There are tight restrictions on religious practice in Saudi Arabia, with religious freedom withheld even from visitors and expats.
Godfrey Yogarajah, executive director of the RLC, said: "Arrest of believers for peacefully gathering for worship goes against the spirit of Saudi Arabia's promotion of inter-religious dialogue in international fora."