The Global Freedom Network, which launched with backing from Most Revd Justin Welby and Pope Francis in March 2014 brings together faith leaders in a commitment to eradicate modern slavery by 2020.
Last week a report claimed up to 13,000 people are living in slavery in Britain, according to the first "scientific" estimate made of the country's modern slave conditions.
The number includes domestic staff, women forced into prostitution and workers in fields, factories and fishing boats - and is four times higher than a previous estimate, by the National Crime Agency's human trafficking centre.
The Modern Slavery Bill, currently going through the UK Parliament, aims to provide courts in England and Wales with new powers to protect people who are trafficked into the countries and held against their will. Scotland and Northern Ireland are planning similar measures.
The Joint Declaration of Religious Leaders against Modern Slavery signed today on the International Day for the Abolition of Slavery underlines that modern slavery, in terms of human trafficking, forced labour and prostitution, organ trafficking, and any relationship that fails to respect the fundamental conviction that all people are equal and have the same freedom and dignity is a crime against humanity, and must be recognised as such by everyone and by all nations.
In an address before the signing, Archbishop Justin described today's declaration as "a profoundly significant moment".
The leaders had gathered to "affirm a deep shared commitment for the liberation of those humiliated, abused and enslaved by their fellow-human beings," he said.
"There are already close and trusting relationships between us as faith leaders. Our task now is to make these relationships work effectively for the well-being of all people."
Archbishop Justin said faith leaders can make sure that every worshiping community knows about modern slavery and is ready to work to prevent and end such abuses.
A number of other faith leaders spoke at the event and video messages were given by His All-Holiness Ecumenical Patriarch Bartholomew and by Grand Ayatollah Sheikh Basheer Hussain al Najafi who could not attend the ceremony but said they were equally committed to eradicating modern slavery and human trafficking.