On the 26 July London demonstration against Israel's assault on Gaza, I confronted a man who was carrying a placard which read “Research: The Protocols of the Learned Elders of Zion”, with an image of a Star of David, dripping blood, with “666” in the centre.

The Protocols are an anti-Semitic forgery dating from Tsarist Russia, which purport to expose a Jewish conspiracy to dominate the world. They were used in their time, and have been used since, to whip up racist hatred, often violent, against Jews.

I told the man that racism had no place on the demonstration, that his presence harmed the Palestinian cause, and that the document he was promoting was a racist hoax. In the course of what was probably a not very coherent tirade from me, I mentioned that I was Jewish.

“Well, you're blinded by your bias because you're a Jew”, he said. “Only Jews make the arguments you're making.”

Thereafter the “discussion” became more heated, and several onlookers were drawn in. Several people backed me up, but several defended him.

Their defences ranged from, “he's opposing Zionists, not Jews”, to “he's not racist, Zionism is racist!”, to the perhaps more honest “Jews are the problem. If you're a Jew, you're racist, you're what we're demonstrating against.” One man, topless, but wearing a balaclava, said “fuck off, unless you want your fucking head kicked in.”

I walked away, angry and upset. I returned a short while later to find the placard-holder embracing two young men, before leaving. When me and some comrades challenged them, they told us he wasn't anti-Semitic, merely anti-Zionist. “Look, it says 'Zion'”, not 'Jews'. 'Zion' means Zionists”, one helpfully informed us.

Explicit anti-Jewish racism of the kind displayed on the man's placard has been rare on Palestine solidarity demonstrations in Britain. But the fact that it was present at all, and that it could find even a handful of defenders in a crowd of other demonstrators, is deeply worrying. Pointing to its rarity, and dismissing the problem as restricted solely to fringe elements, would be to bury one's head in the sand. As recent events in France and Germany have shown, it is an undeniable fact that there are anti-Semites in the global Palestine solidarity movement, and ones prepared to violently express their anti-Semitism. That must not be allowed to infect the movement in Britain.

I don't know how easy a ride the man and his placard had on the demonstration before myself and others confronted him. Had official stewards of the march seen the placard, and challenged him? Perhaps he'd spent all day under attack from other demonstrators; I hope so. But when I found him, he was perfectly at his ease, and, as it turned out, surrounded by friends. That is a disappointment. If people with such politics want to attend solidarity demonstrations to peddle them, they should find themselves isolated, and face constant harangue. They shouldn't be entitled to a moment's peace.

While outward displays of “classical” anti-Semitism are rare, subtler themes are more common. Placards and banners comparing the Israeli state to Nazism, and its occupation of Palestine to the Holocaust, and images melding or replacing the Star of David with swastikas, are, while far from universal, relatively commonplace. The politics of this imagery, too, has an anti-Semitic logic.

Nazism and the Holocaust – an experience of attempted industrialised genocide, just two generations distant – left deep scars on Jewish identity and collective cultural memory and consciousness, wounds that will take a long time to heal. As others have written recently, no other ethno-cultural group has the most traumatic experience in its history exploited in this way. “Zionism = Nazism”, “Star of David = Swastika”, and “The Occupation = The Holocaust” all use collective cultural trauma as a weapon to attack Jews. The fact that those who take such placards on demonstrations intend only to target the Israeli government, and not Jews in general, is no defence or excuse. The barbarism of Israeli state policy does not make the Jewishness of its government fair game, any more than Barack Obama's imperialism excuses racist attacks on him.

To describe the Palestinian solidarity movement, as such, as “anti-Semitic” would be a calumny. Cynics and right-wingers have attempted to use incidents of anti-Semitism to extrapolate conclusions about the politics of all marchers, or to imply that any support for the Palestinians at all is somehow anti-Semitic. Such cynical extrapolations are not my intention with this article. Undoubtedly, the vast majority of marchers attended because they want to oppose Israel's current assault on Gaza. The movement includes many Jews (and not just the theocratic reactionaries of Neturei Karta, but secular-progressive Jews too), and many sincere anti-racists. But a situation where anyone thinks it appropriate to carry such a placard, where he can find supporters, and where such people can openly racially abuse Jewish demonstrators who challenge them, is not tolerable and must be addressed.

Right-wingers in the Jewish community will use instances of anti-Semitism to discredit the Palestinian cause, and dissuade Jews from acting to support it. On this, instrumental, level, anti-Semitism harms the Palestinians. But racism should have no place in any solidarity movement, not because it's bad PR, but because the politics of solidarity should be anathema to any form of racism.

It is now common in the left-wing blogosphere for articles which contain potentially traumatic content to carry “trigger warnings”, alerting those who have experienced particular traumas that something in the article might trigger painful memories of their experience. To attend a demonstration where Nazism and the Holocaust, the worst and most traumatic of Jewish collective experience, is used as a cheap propaganda tool, and openly anti-Semitic placards are carried and defended, while those challenging them are racially abused, must surely be “triggering” for many Jews. But we can't put trigger warnings on demonstrations, or on life. All we can do is work to win hegemony for a political culture where such things are confronted and stamped out.

Finally, a “historical” note on placards on Palestine solidarity demonstrations. In 2009, during Operation Cast Lead, some Workers' Liberty members in Sheffield (three of us, incidentally, Jewish) took placards on a demonstration against the assault which, amongst other things, said “No to IDF, no to Hamas.” As it happens, I now think, for various reasons, that our slogan was misjudged. But no-one attempted to engage us in debate or discussion about it; we were simply screamed at, called (variously) “scabs” and “Zionists”, and told we must immediately leave the demo (we didn't). Our placards were ripped out of our hands and torn to pieces.

As I say, I don't know how many people had challenged the racist placard on the 2014 London demonstration before me; several, I hope. But the political atmosphere on the demo was evidently not such that the man carrying it felt unwelcome – and, indeed, when he was challenged, many people leapt to his defence.

I don't make the comparison in order to express a wish that what happened to us in 2009 had happened to him in 2014. I wouldn't particularly advocate physically destroying the man's placard, or attempting to physically drive him and his supporters off the demonstration. But a movement in which “no to IDF, no to Hamas” is considered beyond the pale even for debate and discussion, and must be violently confronted, but a placard promoting The Protocols of the Learned Elders of Zion can be carried without challenge, even for a moment, and its carrier find numerous defenders, needs to change its political culture.