My first introduction to Popper was in college. I read Objective Knowledge during a course on philosophy of science. Only later--much later--did I learn that he had strong, and controversial, views on social and political issues and a contempt for totalitarianism, and that he felt totalitarianism had its basis in the thought of certain philosophers, among them and perhaps most especially Plato and Hegel. This belief is expressed in his book The Open Society and its Enemies which I've only recently begun to read.
It's a belief I've shared, rather haphazardly, for some time. I've always been dissatisfied with Plato. Perhaps this is the result of having been compelled to read The Republic as a freshman, and then having to spend time discussing it in what seemed at the time annoying detail. It was, I think, my first academic experience in college. Some professor or group of professors had come up with the idea that all freshman should be required to read certain books as part of a course called "Freshman Studies" and The Republic was one of them. I loathed it, and carefully avoided taking the course in ancient philosophy which was part of the philosophical curriculum as a consequence. I suspect that this course was imposed on the philosophy department of the college I attended. Its members were not fond of philosophy which was not Anglo-American.
I was irritated by the "dialogue" style employed by Plato, generally. I later read others of them, and my opinion didn't change. They were stilted and contrived; necessarily, I suppose, but it seemed a most cumbersome and tiresome manner of expression. Oddly, I don't feel as irritated when I encounter this artifice in other writers, such as Cicero, though I don't like it much regardless of the author.
But what was most disturbing to me was the nature of the society envisioned by Plato, which struck me immediately as repressive, and the presumption and arrogance which seemed involved in its formulation. It was unapologetically elitist, and seemed to have its basis in a world-view founded on a curious kind of deductive reasoning the premises of which were in a sense other-worldly and self-justifying. It seemed to have nothing to do with the real world, in fact. Nonetheless, it purported to be based on what was truly real, and good, and of course true.
Popper argues that Plato's metaphysics ultimately requires him to be a totalitarian. He's quite critical of him as a result, but his distaste for Plato is nothing compared with his distaste for Hegel. Plato he sees as a great genius, and it seems he feels his totalitarianism is explicable given the times and circumstances; not admirable, but understandable. Hegel he considers a "charlatan" and worse, and he doesn't hesitate to say so.
He portrays Hegel as an agent (even a venal toady) of Prussian authoritarianism, and he lashes into his philosophy, portraying it as a deliberate perversion of Kant slanted to justify the Prussian, and indeed any other, status quo. The present state--political and otherwise--being the best expression of the Absolute Spirit, or Idea, or whatever, it is necessarily worthy of support, and is indeed the current embodiment of Reason and Freedom, etc., no matter just how repressive it may be. Autocracy is a matter of historical necessity. Well, you get the drift.
I've always found it difficult to read Hegel, but he strikes me as the fountainhead of German romanticism and mysticism one sees ever since, with Fichte and most lately Heidegger. One knows of his influence on Marx and his followers as well, of course. I haven't read far enough yet, but I suspect Popper will argue that Hegel's approach to philosophy was eventually used to justify communism and fascism, especially the Nazi version.
As I understand him, Popper feels that totalitarianism, like the philosophies of those thinkers he criticizes, has its basis in an essentially anti-scientific and irrational point of view, though it strives to cloak itself in science, as it were, and historical necessity. This strikes a chord, I must admit. But I think it is also the case that totalitarian thinkers are also in a sense religious in an institutional sense, or perhaps more properly have personalities similar to those of religious zealots.
There is an awful self-righteousness involved in repression of all kinds; a sureness, an arrogance. And a belief in and desire for certainty, in all things.