Every now and then a proponant of 'tolerance' will cite political philosopher Karl Popper to justify their suppression of the 'intolerant' with:
the paradox of tolerance: Unlimited tolerance must lead to the disappearance of tolerance. If we extend unlimited tolerance even to those who are intolerant, if we are not prepared to defend a tolerant society against the onslaught of the intolerant, then the tolerant will be destroyed, and tolerance with them.
Which certainly strikes as a commonsense conclusion, although it leads one to question the very notion of tolerance. But of course, we should beware common sense, and Popper himself proceeded to gentle the boldness of his claim with some not inconsiderable elaboration.
Karl Popper's first volume of The Open Society and it's Enemies is subtitled The Age of Plato or alternatively The Spell of Plato. The volume is a direct critique of the denouement of thought outlined Plato's The Republic and its widespread adoption in the political thought of the western world. Popper admired the societal problems Plato laid out (e.g. Ch. 5, p. 84), but saw Plato's idyllic notion of all-powerful 'philosopher kings', or 'men of gold' as despotic—a defence of "lying, political miracles, tabooistic superstition, the suppression of truth, and ultimately, brutal violence."(Ch. 10, p. 200).
Popper lays out the 'paradox of tolerance' as one of Plato's motivations for adopting the tyranny of the philosopher-king as an endnote to his chapter on leadership. For Popper, this paradox so concerning to Plato, and indeed all paradoxes that arise in any theory of 'sovereignty' could be solved only under the right conditions of leadership.
Specifically, that democratic political instutions are "reasonably effective safeguards" to these paradoxical dilemmas, but explicitly not "the rule of the people", who under the right conditions never rule in "any concrete, practical sense".
This, then, explains better the context of the rest of Popper's endnote.
In this formulation, I do not imply, for instance, that we should always suppress the utterance of intolerant philosophies; as long as we can counter them by rational argument and keep them in check by public opinion, suppression would certainly be most unwise. But we should claim the right to suppress them if necessary even by force; for it may easily turn out that they are not prepared to meet us on the level of rational argument, but begin by denouncing all argument; they may forbid their followers to listen to rational argument, because it is deceptive, and teach them to answer arguments by the use of their fists or pistols. We should therefore claim, in the name of tolerance, the right not to tolerate the intolerant. We should claim that any movement preaching intolerance places itself outside the law and we should consider incitement to intolerance and persecution as criminal, in the same way as we should consider incitement to murder, or to kidnapping, or to the revival of the slave trade, as criminal.
Popper saw regulation of the intolerant as the role of the same judicial element that regulated crime, limited specifically to those kinds of intolerant expression that suppressed 'rational argument' and instead promoted 'the use of ... fists or pistols'. In this matter, the role of public opinion is not to suppress, but simply to open the floor to debate.
Who would have thought that the first two sentences of a page long endnote in a two volume treatise on democratic ideals would have such nuance around it?