Prostitution is an "evil vice" that reduces women to slavery, often carried out by men who call themselves Christian, Pope Francis wrote in a book preface published in Italian media on Monday.
"All forms of prostitution amount to slavery, a criminal act, a disgusting vice which confuses making love with relieving one's own instincts by torturing a defenceless woman," said the text published in La Repubblica daily.
"A person can never be sold," the pontiff wrote in the preface of a book by Aldo Buonaiuto, a priest attached to the Pope John XXIII Community, a Roman Catholic charity that looks after the poor, prostitutes and troubled.
"It is a sickness of humanity, an erroneous way of considering society. Freeing these poor slaves is a merciful gesture and a duty for all men of good will. Their cry of pain cannot leave individuals or institutions indifferent," the pope wrote.
"No one should look away or wash their hands of the innocent blood spilled on the streets of the world," he said.
The pope also related a visit to one of the community's reception centres where he met "crucified" women and asked their forgiveness for "the tortures they have had to bear because of clients, many of whom call themselves Christian."
