Pope Francis has said "enough" to human ­trafficking, denouncing it as a crime against humanity as police leaders and religious groups from around the world pledged to work together to combat it.

Francis addressed the final session of a two-day Vatican-sponsored international conference on human trafficking attended by top law enforcement officials, politicians and representatives of religions.

"Human trafficking is an open wound on the body of contemporary society, a scourge upon the body of Christ. It is a crime against humanity," he said. Departing from his prepared text, he said there were many "people of good will who want to shout 'enough'" to human trafficking.

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Participants included British Home Secretary Theresa May, Interpol ­Secretary-General Ronald Noble, London Metropolitan Police chief Sir Bernard Hogan-Howe, and anti-­trafficking activists from around the world, many of them nuns.

According to a report last October by the Walk Free Foundation, nearly 30 million people live in ­slavery across the globe, many of them men, women and children trafficked by gangs for sex work and unskilled labour.

Mr Hogan-Howe told reporters the commitment between church groups who work with victims of trafficking and police forces to cooperate was vital. "This is a pretty powerful network," he said.

He added that it was important for leaders like the Pope to make appeals against human trafficking and brand it a crime against humanity because such denunciations could bring concrete results.

"Apart from its mere ­statement, it encourages governments to pass laws. By making such a declaration it encourages governments to take this to a very high priority."

Cardinal Vincent Nichols, the Archbishop of Westminster, said: "We learned that just 1% of people caught in human slavery are being rescued. The comment was made that slavery has never been as widespread in the world as it is today.".