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The Metropolitan Police argued they were legally entitled to obtain, and retain, the information for crime prevention purposes before allowing Susannah Mengesha to leave the kettle.
But Lord Justice Moses and Mr Justice Wyn Williams, sitting in London, ruled they had acted outside their powers.
Ms Mengesha, a law graduate and caseworker at the Bar Pro Bono Unit, was corralled in a crowd of protesters for two hours on November 30 2011 at a trade union march against public sector pension cuts.
The march, which started at Lincoln's Inn Fields proceeded to Piccadilly Circus in the West End. Some of the crowd, including Ms Mengesha, were blocked in at nearby Panton Street. Police said the containment was "necessitated by a reasonably apprehended imminent breach of the peace".