Migrants are transported to detention center in coastal city of Sabratha, Libya in October
The demand follows the release of CNN footage of a live auction in Libya where black youths are presented to North African buyers as potential farmhands and sold for as little as $400.
Guinean President Alpha Conde, who is also chairman of the African Union, demanded an enquiry and prosecutions relating to what he termed a "despicable trade... from another era".
Meanwhile, Senegal's government, commenting on Facebook, expressed "outrage at the sale of Sub-Saharan African migrants on Libyan soil," which constituted a "blight on the conscience of humanity".
African migrants from nations including Guinea and Senegal, but also Mali, Niger, Nigeria and Gambia make the dangerous crossing through the Sahara to Libya with hopes of making it over the Mediterranean Sea to Italy.
But testimony collected by AFP in recent years has revealed a litany of rights abuses at the hands of gangsters, human traffickers and Libyan security forces, while many end up stuck in the unstable North African nation for years.
Recent investigations by Middle East Eye revealed that armed groups have been stopping migrant boats from leaving Libya in exchange for aid, aircraft hangars and large sums of money. People say that European intelligence agencies have been negotiating with local militias to prevent boats from heading for Italy.
A spokesperson for the Italian prime minister, Filippo Sensi, told MEE that the government does not comment on anonymous allegations.
Meanwhile, more than 8,800 stranded migrants have been returned home this year, according to the International Organization for Migration, which is also amassing evidence of slavery.
Conde further appealed for the Libyan authorities to "reassess migrants' detention conditions" following revelations over squalid jails and detention centres that await migrants who are caught trying to reach the coast.
"These modern slavery practices must end and the African Union will use all the tools at its disposal," Conde added.
Libya has opened an investigation into the practice, CNN reported on Friday, and pledged to return those taken as slaves to their country of origin.