US 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.
"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 told reporters.
"I am horrified at news reports and video footage showing African migrants in Libya reportedly being sold as slaves," he said, adding: "I abhor these appalling acts."
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."
Libyan Deputy Prime Minister Ahmed Metig said his UN-backed Government of National Accord would investigate the allegations, in a statement posted Sunday on the Facebook page of the GNA's press office.
Guterres wants Libyan authorities as well as the International Criminal Court, which has a mandate to open war crimes investigations in Libya, to look into the slave auctions, said UN spokesman Farhan Haq.