According to Christian Solidarity Worldwide (CSW), the women were detained in a series of raids on members of non-sanctioned churches by the Eritrean military, which began in May.
The 33 women, mostly young mothers, were among the first to be arrested and are being held in Nakura, a notoriously harsh prison island in the Dahlak Archipelago that was created by Italian colonialists in the late 19th Century to crush political dissent.
The raids are the latest phase in a clampdown on Christians that has been ongoing since May 2002, when the Eritrean government effectively outlawed religious practices not affiliated with the Catholic, Evangelical Lutheran and Orthodox Christian denominations or Sunni Islam.
CSW described the situation as "unprecedented in its intensity and rough treatment" against Christians. Adding "Christians across the country have taken to their knees in expectation of God's faithfulness and grace in these difficult times and call on others who read of their plight to join them in prayers."
CSW's Chief Executive Mervyn Thomas said: "This spike in arrests provides a clear illustration that the persecution of unrecognised faith groups in Eritrea is continuing.
"Moreover, the detention of these women, many of whom are young mothers, in a notoriously harsh facility simply on account of their religion or belief, is unwarranted, deplorable and in violation of Eritrea's obligations under international law.
CSW has welcomed the renewal of the Human Rights Council (HRC) resolution which extends a mandate that will ensure continued human rights monitoring in Eritrea.
Thomas added: "It is time to prioritise accountability for human rights violations; thus we reiterate our call for the international community to facilitate justice for victims of atrocity crimes, and to maintain pressure on the Eritrean regime until every prisoner of conscience is freed without precondition."
