Anti-Israel campaigners have every right to assemble, demonstrate, picket and argue. But preventing speech that they disapprove of is no legitimate part of a democratic society. Shouting down a musical performance is the same type of outrage, but a degree worse. It is not mere boorishness but militant philistinism. Its near equivalent is book-burning.

The 30 or so demonstrators who purported to be advocating Palestinian national rights (a principle of justice accepted by successive Israeli governments and widely shared among Israeli voters) damaged their own cause and doubtless magnified the deserved ovation within the Albert Hall for the IPO.

But ineffectuality does not mitigate intolerance. Often, as with the small groups of revolutionary leftists of the 1970s, the sense of being part of an elect only intensifies assaults on liberty. A responsibility in these circumstances lies with government, law enforcers, pundits and the public. It is our first duty to defend the right to self-expression. To do that requires us to be intolerant of the intolerant.

The familiar street protests are about what Israel represents: Jewish nationhood and constitutional democracy. Boycotts of Israeli academics and cultural organisations - none of whom controls Israeli government policies, which they may disagree with - are an assured way of coarsening British public life and inflaming political debate. But the attack on the IPO is beyond this. It reflected a prior determination that an Israeli orchestra, purely because of its members' nationality, would not be heard.

In short, the protest exemplified bigotry and xenophobia. In withstanding it, the IPO defended more than Israel's reputation alone.

Oliver Kamm is a leader writer for The Times.