St Patrick's Cathedral agreed to have the ribbon around the iconic building to mark the launch of the 'flesh and blood campaign'.
The two Archbishops of Armagh gathered with healthcare experts from Northern Ireland and the Republic to urge people to help.
Archbishop Richard Clarke from the Church of Ireland said: "The Church is an organism that is characterised by its giving nature; its people are characterised by being giving people.
"So, when there is need across Ireland for more people who would give the gift of blood in life, or of organs after earthly death, this is an opportunity for giving of which all should become aware."
Archbishop Eamon Martin, from the Catholic Church, said: "The challenge of the flesh and blood campaign, to see blood and organ donation as a part of our giving within the Church, is one I encourage us all to consider; that we might mirror God's perfect gift to us in His Son by giving the gift of life ourselves."
Dr Eddie Rooney, Chief Executive, Public Health Agency in Northern Ireland, said: "In Northern Ireland and the Republic, around 800 people are currently waiting for an organ transplant, with 96,000 people receiving blood transfusions every year.
"We hope that many more lives will be saved and transformed through the generous commitment made by the Churches in Ireland to raise the profile of blood and organ donation, placing it on the agenda for the Church."