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.
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.".