More than 500 people were arrested in central London on Saturday after a protest in Trafalgar Square against Britain’s ban on Palestine Action, a pro-Palestinian activist group.

The Metropolitan Police said 523 individuals were detained for showing support for a proscribed organisation.

The gathering was the first of its kind since London’s High Court ruled in February that the government’s designation of Palestine Action as a terrorist organisation was unlawful.

That ruling was a significant rebuke to Shabana Mahmood, the interior minister, who has since been granted leave to appeal. Until that appeal is decided, the ban sits in a legal grey zone, making even peaceful expressions of solidarity a potentially criminal act.

Critics argue that the mass arrests represent a troubling use of counter-terrorism powers against political speech.