London is much reviled by people who don’t live there. It has its share of social problems typical to a large metropolis, but it is unusual in having also a dystopian twin – a fallen city, overrun with violent criminals, located in the imagination of rightwing politicians and the online sources they consume.

The capital’s denigrators felt vindicated recently by scenes of disorder on Clapham High Street. Hundreds of young people, rallied on social media, mustered for a spontaneous gathering, which degenerated into a spree of antisocial behaviour. Images of the disorder were shared online as proof of the capital’s status as a no-go area. Nigel Farage, the leader of Reform UK, said that the unrest was symptomatic of “societal breakdown”.

Sadiq Khan, London’s mayor,........

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