Summit host Ouattara lashed slavery as a "wretched drama which recalls the worst hours of human history."
"I would like to appeal to our sense of responsibility to take all urgent measures to put an end to that practice, which belongs to another age," he said, opening the gathering of 55 African Union and 28 European Union nations in Ivory Coast's economic capital.
An emergency meeting on the issue was scheduled for Wednesday night, bringing together France, Niger, Chad and Morocco as well as UN, EU and AU officials, the office of French President Emmanuel Macron said.
The summit comes just two weeks after US network CNN aired footage of black Africans sold as slaves in Libya, sparking outrage from political leaders and street protests in African and European capitals.
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African Union and other critics have accused the EU of creating conditions for the slave trade as well as rape and torture of migrants by encouraging Libya's UN-backed Government of National Accord (GNA) to detain migrants and stop them from coming to Europe.
In his speech to the summit, European Council President Donald Tusk acknowledged the "horrifying" treatment of young Africans.
But he also warned against starting "a blame game" and called for cooperation to fight criminals.
Amnesty International accused Europe of being "complicit" in the abuses by funding, training and instructing the Libyans to detain migrants.
The group instead called on the African leaders to urge Europe to open more safe and legal migrant routes to Europe.
The "gross and scandalous" auctions made "mockeries of the alleged solidarity of African nations grouped in the African Union, of which Libya is a member," he said.
Separately, Nigerian President Muhammadu Buhari, in a statement issued by his office, said it was appalling that his compatriots were being sold "like goats".
He vowed to repatriate and rehabilitate Nigerians stranded in Libya, and reduce the numbers heading for Europe illegally via North Africa.