Portuguese President Marcelo Rebelo de Sousa said late on Tuesday that Portugal was responsible for crimes committed during transatlantic slavery and the colonial era, and suggested there was a need for reparations.

For over four centuries, at least 12.5-million Africans were kidnapped, forcibly transported long distances by mainly European ships and merchants, and sold into slavery.

Those who survived the voyage ended up toiling on plantations in the Americas, mostly in Brazil and the Caribbean, while others profited from their labour.

Portugal trafficked nearly 6-million Africans, more than any other European nation, but has failed so far to confront its past and little is taught about its role in transatlantic slavery in schools.

Instead, Portugal's colonial era, during which countries including Angola, Mozambique, Brazil, Cape Verde and East Timor as well as parts of India were subjected to Portuguese rule, is often perceived as a source of pride.

Speaking at an event with foreign correspondents late on Tuesday, Rebelo de Sousa said Portugal "takes full responsibility" for the wrongs of the past and that those crimes, including colonial massacres, had "costs".

"We have to pay the costs," he said. "Are there actions that were not punished and those responsible were not arrested? Are there goods that were looted and not returned? Let's see how we can repair this."

The idea of paying reparations or making other amends for transatlantic slavery has been gaining momentum worldwide, including efforts to establish a special tribunal on the issue.

Activists have said that reparations and public policies to fight inequalities caused by Portugal's past, including systemic racism, are essential.

Rebelo de Sousa said last year that Portugal should apologise for transatlantic slavery and colonialism but stopped short of a full apology. He said on Tuesday that acknowledging the past and taking responsibility for it was more important than apologising.

"Apologising is the easy part," he said.