Since 2002, the government has only recognised four religions
Thirty elderly women have been arrested in Eritrea while praying together, one of their relatives living in the United States has told the BBC.
Requesting anonymity, she said she only found out about her mother when she phoned to speak to her this week.
Most of the women belonged to an outlawed evangelical group.
The government recognises only four faiths - Islam, Orthodox, Catholic and Lutheran - and bans gatherings of more than five people.
Eritrea's information minister told Reuters news agency he was not aware of the weekend arrests.
"If they were arrested, I'm sure they were committing a crime," Ali Abdu is quoted as saying.
Human Rights groups regard Eritrea as one of the world's most repressive states.
'No security risk'
"Nobody's seen them. She's not allowed to carry a cell phone," said the daughter of one of the women taken into custody at the prayer meeting in the capital, Asmara.
She told the BBC's Network Africa programme that her mother met friends to read the Bible and pray every weekend.
A Christian religious rights group in the US has condemned the arrests and expressed concern for the women's safety.
"We know that Christians who are imprisoned in Eritrea are mistreated, they face torture," International Christians Concern's Jonathan Racho said.
"They don't pose any security risk to the country," he said.
According to a recent US State Department report, more than 3,000 Christians from outlawed denominations have been detained since the religious crackdown began in 2002.
More than 90% of Eritreans belong to one of four recognised faiths.