Y Care International is calling on the public to lobby the Israeli government for the release of a member of staff from the East Jerusalem YMCA.
Haytham Salah Al-Hammouri is being held without trial in an interrogation centre and has been placed under administrative detention for six months.
He was taken away after armed Israeli police entered the East Jerusalem YMCA building and checked staff and visitor ID cards.
He is believed to have been detained because of visits to the towns of Ramallah and Bethlehem on the West Bank where the YMCA runs vocational training and rehabilitation programmes funded by Y Care International.
Dr Chris Beer, director of Y Care International, has returned from a visit to the West Bank and Gaza and has expressed deep concern over the safety and welfare of staff and young people on East Jerusalem YMCA programmes.
Judeh Majaj, general secretary of East Jerusalem YMCA, said: "Haytham's administrative detention is a violation of the Universal Declaration of Human Rights and a clear obstacle to our efforts to work with Palestinian people through these difficult times."