Police said on Saturday they had arrested 523 people at a protest in London's Trafalgar Square opposing Britain's banning of the Palestine Action group.
London's Metropolitan Police said in a post on X that people were arrested for showing support for a "proscribed organization."
The gathering was the first since London's High Court ruled in February that a ban designating the pro-Palestinian group as a terrorist organization was unlawful. UK Interior Minister Shabana Mahmood has been granted approval to appeal the ruling.
Palestine Action banned after air force base break-in
Lawmakers banned Palestine Action - which accuses Britain's government of complicity in what it says are Israeli war crimes in Gaza - under anti-terrorism legislation last July after some members broke into a Royal Air Force base.
Israel has repeatedly denied any abuses in its war in Gaza.
During Saturday's demonstration, placard-holding protesters, some wearing black and white Palestinian scarves and waving Palestinian flags, sat on the ground or on camping chairs.