From the 15th to the 19th century, 6 million Africans were kidnapped and forcibly transported across the Atlantic by Portuguese vessels and sold into slavery, primarily to Brazil.
But so far Portugal has rarely commented on its past and little is taught about its role in slavery in schools.
Rather, the country's colonial era, which saw countries including Angola, Mozambique, Brazil, Cape Verde, East Timor as well as parts of India subjected to Portuguese rule, is often perceived as a source of pride by most Portuguese.
Speaking at Portugal's annual commemoration of the 1974 "Carnation" revolution, which toppled the country's dictatorship, Rebelo de Sousa said the country should go beyond just an apology, though he did not offer up any specifics.
"Apologizing is sometimes the easiest thing to do: you apologize, turn your back, and the job is done," he said, adding the country should "assume responsibility" for its past to build a better future.
Rebelo de Sousa made the remarks after Brazilian President Luiz Inacio Lula da Silva, who was in Portugal on his first visit to Europe since assuming the office, addressed the Portuguese parliament. Brazil gained independence from Portugal
in 1822.
He said the colonization of Brazil also had positive factors, such as the spread of Portuguese language and culture.
"(But) on the bad side, the exploitation of Indigenous people...slavery, the sacrifice of the interests of Brazil and Brazilians," he said.
Europe's top human rights group previously said Portugal had do more to confront its colonial past and role in the transatlantic slave trade in order to help fight racism and discrimination today.
