The service at St. Peter's Basilica marks the Roman Catholic Church's World Day of Peace. This year's theme is "No Longer Slaves, but Brothers and Sisters."
"All of us are called (by God) to be free, all are called to be sons and daughters, and each, according to his or her own responsibilities, is called to combat modern forms of enslavement. From every people, culture and religion, let us join our forces," he said.
Last month Francis appealed to consumers to shun low-cost goods that may be the product of forced labor or other forms of exploitation.
That message was sent to heads of state and governments, international institutions and parishes throughout the 1.2 billion-member Church.
The Argentine pope has made defense of migrants and workers a central issue of his papacy. At a Vespers service on New Year's Eve, he condemned administrators and criminals in Rome accused of pocketing public funds meant to help poor migrants, urging a "spiritual and moral renewal."
The second global slavery index released in November by the Walk Free Foundation, an Australian-based human rights group, estimated that almost 36 million people were living as slaves, trafficked into brothels, forced into manual labor, victims of debt bondage or born into servitude.
After Mass, the pope delivered his traditional New Year's Day noon address to tens of thousands of people, most of whom took part in peace marches to the Vatican.
"Peace is always possible but we have to seek it. Let us pray for peace," he told the crowd of people carrying balloons and banners with peace slogans