An old song has been coming back to me, one I learned back in my youth movement days. Perhaps it's in my mind because, last week, I packed off my eldest son to his first summer camp.

The song is Gesher Tzar Me'od (A Very Narrow Bridge) and it resonates now because that is where I feel I stand - on a very narrow bridge, getting narrower by the day.

On July 10, I took part in a debate at the South Bank Centre that was part of the London Literature Festival. Its theme was the rights and wrongs of cultural boycotts in general, with an inevitable focus on the proposed boycott of Israel.

This event has already been widely discussed in the JC, including one story with the headline, Freedland gloom as Israel boycott is applauded.

It's quite true that I found it a gloomy experience. Partly because the event was, in effect, jointly organised by British Writers in Support of Palestine - a fact that emerged only later but which the South Bank Centre did not disclose to me or my fellow anti-boycott panellist, Carol Gould, and which it did not reveal in its publicity material. The audience was woefully one-sided, consisting almost entirely of committed boycotters of Israel. At times the atmosphere got pretty nasty: there was repeated jeering, booing and the odd obscene hand gesture from assorted members of the audience.

Israel's boycott law seems designed to confirm its enemies' views Jonathan Freedland

Still, I was not, despite what the JC said later, "visibly shaken". I've appeared in front of similar audiences before and my skin has thickened. (Brief tangent on that point: I wonder how many of those bloggers and JC letter-writers who frequently denounce me as insufficiently "pro-Israel" regularly defend the country, not from their armchair or at cosy gatherings of like-minded Israel supporters such as the recent We Believe conference, but in front of Israel's most strident opponents. Certainly not one of them turned up at the South Bank to oppose the boycott. Next time they call me a traitor to Israel or worse, remind me to ask them where they were on July 10.)

For all that, I did find the event useful. What it confirmed out loud was that the hard core of boycott campaigners do not merely object to the post-1967 occupation- even if that dominates their public rhetoric - but to Israel as Israel. Speakers from the floor repeatedly returned to the alleged ills of pre-1967 Israel and of Zionism itself. Indeed, Naomi Foyle, the activist who had acted as a "volunteer consultant" to the South Bank in organising the debate, later blogged a concise response to my claim that the boycott campaign was anti-Israel rather than anti-occupation: "Damn right."

I think it's helpful that the boycotters are exposed in this way. Because many of those tempted to heed the boycott call - and it's important to distinguish followers from leaders - will be drawn to it as a way to oppose the occupation. Some, not all, will be less keen to join a campaign hostile to Israel's very right to exist.

So I was happy to stand against the boycott. But guess what happened a few days later. Israel passed an anti-boycott law that seemed designed to confirm everything the country's enemies say about it.

A grotesque violation of the basic right of free speech, it makes it illegal not just for an Israeli living in Tel Aviv to boycott, say, goods produced in the West Bank but even to advocate such an idea. At a stroke, it undermines Israel's repeated claim to be "the only democracy in the Middle East".

This is what I mean about standing on a very narrow bridge. On one side are the Israel-haters. On the other are those leading Israel into an ever darker place, backed by allies abroad who cheer them on, almost never saying enough is enough. To their credit, many did speak out against the anti-boycott law – but that proposal is not a one-off. The Knesset is now debating a plan to drop Arabic as an official language, even though it is the mother tongue of one fifth of the population and has been respected as such since the day the state was founded.

So, yes, I condemn the boycott, but I also condemn the boycott law. I deplore Israel's enemies, but I also deplore acts of madness like this.

And though the bridge feels so narrow, I suspect there are many who stand in exactly the same place.