He said that exploiting the working people to enrich a person is "like sucking blood."
The exploitation of labour is a mortal sin, he said.
He was speaking during morning Mass at the Casa Santa Marta where he criticised the "theology of prosperity".
The pontiff told the audience "you cannot serve both God and riches."
Pope Francis in Lesbos in spring 2016
Pope Francis has often been outspoken and defended the poor, during his 2016 visit to Mexico he told bishops to better protect local people from corruption and the attractions of a gang lifestyle.
Vatican Radio also reported that Pope Francis told a story of a young girl who told him about working 11 hours a day for €650 a month (around £500).
Her employer hold her take it or leave it, because there were more people that wanted the job.
These rich people, the pope said, "grow fat on their riches."
He finished by saying that people think "that slaves no longer exist: they exist.
"It's true, people no longer go to Africa to capture them in order to sell them in America, no.
"But it is in our cities. And there are these traffickers, these people who treat the working people without justice."
