In the good police procedural, the world-weary sergeant briefs officers on shift as to their priorities for the day: miscreants to watch out for, activities that deserve the blind eye, those that should be dealt with. If the ritual plays out in east London today, the briefing should be relatively simple: find out who flew a jihadist-style flag above a municipal housing estate in Tower Hamlets – find them fast and throw the book at them.

Perhaps it was a prank. But if so, it was a well-thought-through piece of mischief. The emblem, on top of the gates of the Will Crooks estate, on Poplar High Street, was surrounded by flags of Palestine and slogans.

Perhaps it was intended as a protest,a gesture of solidarity with those suffering terrible horrors in Gaza. Anyone who has seen a news bulletin must be touched and appalled by the plight of those caught up in a dreadful, murderous conflict.

And, as a principle, one always defends the right to protest. That right is one of the cornerstones of our free society. But the effect of this goes beyond that: it’s about shared space.

The thinking behind flying such a flag at the gates of a housing estate is to say something about the estate. It is to mark territory. Consider the reaction when journalists from the Guardian were spotted examining and photographing the installation. Twenty youths quickly emerged, swore at them and ordered them to leave the area. Earlier, a man seen photographing the structure was challenged to state whether he was Jewish. Would it make a difference, he asked. The reply: “Yes, it fucking would.”

Quite apart from the yobbishness of this, there is an overweening arrogance on display that should not be tolerated. The public realm is just that: it belongs to everyone. The mob may or may not reside on the Will Crooks estate, but they certainly don’t own it. It is a public facility built on communally owned land. They have a right to be there, but so do I, and so does anyone else who chooses to assert that right, so long as they obey the laws we have framed to regulate good behaviour in our society.

We must be intolerant to the erosion of our rights in shared space. It is bad enough that so much space is now privatised – gated and reserved for those who can buy a measure of seclusion from the rest of us. If we stand by and allow those whose idea of ownership so conflicts with the tenets of free society to ringfence our shared space, we make a bad situation worse.

We argue about British values, but without doubt one of the qualities we aim for is tolerance. In modern Britain, this is a prerequisite. Our cities are home to a vast array of people, with backgrounds from every continent. In the capital, according to the last census, at least 100 languages are spoken in almost every borough. Cities exhibit diversity in terms of background, religion and sexuality. The potential for conflict is endless; that so little conflict actually occurs is one of the joys of living in 21st-century Britain.

But one of the reasons for that is shared space. In London, where that diversity is most concentrated, most communities are shared and mixed. Some districts have groups that seem dominant by dint of numbers. Sometimes we celebrate that: think Banglatown, Chinatown and the part of South Kensington that presents as a suburb of Paris. But generally we are wary of racial, religious or cultural Balkanisation.

The rationale behind those who would claim the Will Crooks estate for their worldview is Balkanisation. They are not alone. Others in recent memory have proclaimed where they live to be “gay-free zones”. There are zealots who have taken it upon themselves to harass people drinking alcohol, or women they see as immodestly dressed. This displays a baleful mindset, a complete misunderstanding of how we lived in shared space. We should leave no room for doubt: we are intolerant of such intolerance.