Since Charlottesville, I have seen many references to this by people looking to justify, or cast in a positive light, violence by AntiFa groups and their affiliates. It would be useful for people to read the entire "The Open Society and Its Enemies"[1]. The "Paradox of Tolerance" appears in Note 4 to Chapter 7:

Less well known is 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.—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. (emphasis mine)

which is less than a full-throated defense of violence against people whose speech we find disgusting and reprehensible.

The context in which the note is referenced is this:

One particular form of this logical argument is
directed against a too naïve version of
liberalism, of democracy, and of the principle
that the majority should rule; and it is
somewhat similar to the well-known ‘paradox of
freedom’ which has been used first, and with
success, by Plato. In his criticism of
democracy, and in his story of the rise of the
tyrant, Plato raises implicitly the following
question: What if it is the will of the people
that they should not rule, but a tyrant instead?
The free man, Plato suggests, may exercise his
absolute freedom, first by defying the laws and
ultimately by defying freedom itself and by
clamouring for a tyrant[4].

That is, Popper sees paradoxes of freedom and tolerance as related. Later, he resolves this like Kant before him did:

I believe that the injustice and inhumanity of
the unrestrained ‘capitalist system’ described
by Marx cannot be questioned; but it can be
interpreted in terms of what we called, in a
previous chapter[20], the paradox of freedom.
Freedom, we have seen, defeats itself, if it is
unlimited. Unlimited freedom means that a strong
man is free to bully one who is weak and to rob
him of his freedom. This is why we demand that
the state should limit freedom to a certain
extent, so that everyone’s freedom is protected
by law. Nobody should be at the **mercy** of others,
but all should have a **right** to be protected by
the state.

I have always been particularly fond of the conclusion:

Instead of posing as prophets we must become the
makers of our fate. We must learn to do things
as well as we can, and to look out for our
mistakes. And when we have dropped the idea that
the history of power will be our judge, when we
have given up worrying whether or not history
will justify us, then one day perhaps we may
succeed in getting power under control. In this
way we may even justify history, in our turn. It
badly needs a justification.

[1]: https://archive.org/details/TheOpenSocietyAndItsEnemiesPoppe...