Pope Francis surprised 20 former prostitutes on Friday by knocking on their door in Rome and popping in for a chat.

The Argentine, who has repeatedly described the human trafficking behind much prostitution as 'a crime against humanity', sat down with the group, including four women from Albania, seven from Nigeria and six from Romania.

The other three women - who are being sheltered by a Catholic charity in an apartment - hailed from Italy, Tunisia and the Ukraine.

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Pope Francis surprised 20 former prostitutes on Friday by knocking on their door in Rome and popping in for a chat. He is pictured here greeting one of the women

The 79-year old listened for over an hour to the stories of the former sex slaves, who are living under protection

The women, all aged around 30, are being sheltered by the the Papa Giovanni XXIII community in an apartment in the Italian capital after being rescued from their pimps

The women, all aged around 30, are being sheltered by a Catholic association in an apartment in the Italian capital after being rescued from their pimps.

The 79-year old listened for over an hour to the stories of the former sex slaves, 'all of whom suffered serious physical abuse and live under protection,' the Vatican said.

The visit fell under what have been termed Francis's 'Fridays of Mercy', whereby he carries out one unscheduled act of mercy a month on a Friday.

These take place mainly in or near Rome throughout the pope's Jubilee year, which started in December and runs to November.

The Argentinian pontiff kisses a baby during a visit to the centre. His visit fell under what have been termed Francis's 'Fridays of Mercy'

As part of the initiative, the pope carries out one act of mercy a month on a Friday. In this photo, he is handed a rosary during the visit to the Papa Giovanni XXIII community

Pope Francis greets one of the association's residents. The charity was founded in the seventies and also works with disabled people

In January, he visited a care home for the elderly and people in a vegetative state, while in February he went to a community for drug addicts.

In March he toured a refugee centre before visiting asylum seekers on the Greek island of Lesbos in April.

The pontiff then spent time with the seriously mentally ill in May before visiting old and ill priests in June.

And he dedicated his July 'Friday of Mercy' to sick children in Krakow after praying for the victims of the Holocaust at the Auschwitz-Birkenau Nazi death camp.

Popel Francis speaks to the former sex slaves. In March he toured a refugee centre before visiting asylum seekers on the Greek island of Lesbos in April

The pontiff then spent time with the seriously mentally ill in May before visiting old and ill priests in June