Some 150 people were arrested in London on Saturday during a tense protest in support of the Palestine Action group, which has been banned under terror laws, police said.
Several hundred people demonstrated in front of the UK parliament, with some holding placards that read: "I oppose genocide. I support Palestine Action."
The capital's Metropolitan Police force (Met) had warned people that it would not hesitate to arrest anyone who explicitly expressed support for the prohibited group.
"We are not terrorists," 74-year-old retiree Polly Smith told AFP, adding: "The ban must be lifted."
Nigel, a 62-year-old CEO of a recycling company who declined to give his surname, said the government's ban imposed in July was "totally inappropriate".
"They should spend more time working on trying to