There was large scale slavery of Europeans in the Ottoman Empire, along with North Africans and Caucasians (people from the Caucasus) to the point where in the early 17th century up to 1/5 of the population of Constantinople were slaves. The reasons for slavery were numerous, but the most common sorts of slaves were military slaves and sexual slaves. The Ottoman Imperial harem was largely comprised of slave women captured from what is modern Russia and Ukraine on the northern coast of the Black Sea, one of the most famous women in Ottoman history was Roxelana (legally: Hürrem Sultan) who was born an Orthodox Christian in the city of Rohatyn in what is now the Ivano-Frankivsk Oblast of Ukraine. In the 1520s she was kidnapped by Crimean Tatars and eventually taken to the slave market in Constantinople where she was selected for the harem of Suleiman "the Magnificent" and was at first a consort but rose to the status of legal wife by giving birth to his heir. From this status she gained unprecedented power and influence over Ottoman affairs and is one of the most powerful women in the history of the Ottoman Empire.

The Ambassador of the Duchy of Lithuania (which together with Poland ruled a large portion of modern day Ukraine) to the Crimean Peninsula wrote

"[..]the most beloved wife of the present Turkish emperor – mother of his primogenital [son] who will govern after him, was kidnapped from our land"."

Another widespread form of slavery was via the devsirme system of forced conscription of Christian boys from the Balkans. This began in the 14th century and led to the creation of the Jannissary corps, the Jannisaries were an elite infantry unit of slaves under the direct authority of the Sultan and bodyguards of the Imperial household. The Jannisaries were kidnapped at a young age, or voluntarily given by their families and forcibly converted to Islam. From that point they were trained and educated to be disciplined soldiers, and unlike other slaves they were paid salaries. They were forbidden to marry or engage in trade, as the Sultan wanted their loyalty and devotion to be solely to him and not women or wealth. The Jannisary corps was abolished and 6000 of them were executed after a mass revolt against Sultan Mahmud II. The conditions preceding the revolt were primarily the growth of the power and influence of the Jannisary corps as a disciplined warrior caste, who via implicit threat held a veto over the affairs of the Sublime Porte.

There are others, like the raids in the Western Mediterranean that led to the Barbary Wars but I'm not nearly as knowledgeable. One sticking point: White people as you know them are a very modern American concept. The idea of Russian and Balkan Slavs, Italians and Frenchmen being the same race would've not only been ludicrous but repellent to most of those people just a century or so ago.