London’s Metropolitan Police said they detained 532 people during a sit-down protest in Parliament Square on Saturday after demonstrators displayed placards supporting Palestine Action, a group proscribed under the Terrorism Act last month. Of those held, 522 were arrested for expressing support for a banned organisation and 10 on separate charges including assaulting officers; no serious injuries were reported. Officers moved in minutes after hundreds of protesters simultaneously raised signs reading “I oppose genocide. I support Palestine Action”. The crowd, estimated by police at 500-600 people, had been mobilised by the campaign group Defend Our Juries to test the new legislation. Detainees who confirmed their identities were later bailed on condition they avoid further rallies for the group. Parliament outlawed Palestine Action in July after activists broke into RAF Brize Norton and caused damage to two Voyager tanker aircraft. Under the Terrorism Act 2000, membership or public support for the organisation carries a maximum 14-year prison sentence. The group’s co-founder, Huda Ammori, has won permission to mount a High Court challenge to the ban, arguing it violates free-speech protections. Saturday’s operation was the largest single-day mass arrest by a UK police force in at least a decade, according to police data. The crackdown came ahead of a separate march on Sunday in which pro-Israel demonstrators called for the release of hostages held in Gaza, underscoring the heightened tensions Britain faces as the Middle East conflict continues.