Guterres called on "all competent authorities" to investigate the slave auctions without delay, adding that he had asked the "relevant United Nations actors to actively pursue this matter".
"Slavery has no place in our world, and these actions are among the most egregious abuses of human rights and may amount to crimes against humanity", Guterres said in a statement.
United States television network CNN aired the footage last week of an apparent live auction in Libya where black men were presented to North African buyers as potential farmhands and sold off for as little as $400.
Libyan Interior Minister of the UN-backed government, Al-Aref Al-Khoja, on Thursday said the government has improved living conditions of the country's immigration housing centers. "We are now currently waiting for the results of the investigations which I believe are coming to a close".
As they have before, Libyan officials have stressed they are overwhelmed by the numbers taking advantage of their location across from southwestern Europe.
"What happened in Libya is shocking, scandalous, but we must establish the responsibilities", Conde said.
This summer, Italy pledged to help Libyan officials to patrol the seas off their coast. Many others have reached Italy or been rescued on the way.
The operation, dubbed Sophia, was created two years ago to combat human trafficking in the Mediterranean, and has rescued about 42,000 people attempting the perilous crossing from Africa to Europe since then. The global police organization said 236 minors were included in the total rescued in simultaneous operations across Chad, Mali, Mauritania, Niger and Senegal. "The Libyan Government must address comprehensively this outrage to the conscience of humanity".
Those arrested face prosecution for offences including human trafficking, forced labour and child exploitation.
Trading in human beings as witnessed recently in Libya is a "crime against humanity", the chairman of the African Union (AU) Commission said Tuesday after a recent revelation of a slave trade in Libya shocked the global community.
"Given Rwanda's political philosophy and our own history, we can not remain silent when human beings are being mistreated and auctioned off like cattle", Foreign Minister Louise Mushikiwabo told German news agency DPA. As Europeans we must deal with this dramatic situation that our African brothers and sisters are experiencing. Her country for almost a quarter-century has been recovering from its own tragedy, the 1994 genocide that left around 800,000 people dead.