Christians are being encouraged to write to the diplomatic building on White Lion Street in north London and call for the release of believers imprisoned for their faith.
Kate Ward from Release International - which is behind the Pray 173 initiative - said: "That [the postcard] is sent off to [the Eritrean] government to say 'we, as the Church in the UK, implore you to release those who are in prison."
During the course of the summer and autumn months, 173 prayers are being issued by Release International to highlight the plight of persecuted believers in Eritrea.
Kate Ward explained the significance of that figure, saying: "There are at least 173 Christians in prison who have been held, on average, for more than ten years."
Eritrea is among the top ten countries in the world where persecution against Christians is most severe, according to Open Doors.
The Christian organisation cites dictatorial paranoia and Islamic extremism as the main sources of persecution against believers - who comprise 2.7 million of Eritrea's five million residents.
Release International, which serves persecuted Christians in 30 countries around the world, reports that thousands of people have been imprisoned for their faith in Eritrea since 2002.
Evangelical and Pentecostal churches have been outlawed there for the last 15 years, the organisation claims.