"Civilized" is a loaded word to use when talking about the complexity of a society or a culture.
The short answer to your first question is "yes;" the societies in Egypt, Kush/Nubia, North African coast (including Carthage, which was started as a Phoenician colony) and Jenne-Jeno along the Niger River were highly sophisticated. The medieval societies in Ghana, Mali, Great Zimbabwe and the Swahili coast were also sophisticated and were in close contact with the Eurasian "civilizations" of the time.
Your first question makes some huge assumptions, though. First, what we can call "slavery" was almost always a feature of these, and other, ancient societies. Second, it suggests that the Transatlantic slave trade that depopulated Africa to supply slave markets in the Americas and in European and Asian kingdoms was somehow a "civilizing" influence, which the historical record shows is not the case.
Your second question is easier. Egypt, 'nuff said.
Some good sources:
John Reader, _Africa: A Biography of the Continent_. This book is thick and might be intimidating but is well worth the time spent reading it.
Thomas Pakenham's _The Scramble For Africa_ is another thick book but a good demonstration about just how UNcivilized the 'civilized' Western Europeans were to Africans in the late 1800s and early 20th century.
Henry Louis Gates documentary series on the "Wonders of the African World" is controversial, but it provides an American perspective on modern African countries and their historical legacy.
Time-Life video's "Lost Civilizations" series has an episode on African cultures and how many Western Europeans and others have tried to suppress or, frankly, lie about Africans' histories.