Britain banned Palestine Action under anti-terrorism legislation in July after some of its members broke into a Royal Air Force base and damaged military planes. The group accuses Britain’s government of complicity in what it says are Israeli war crimes in Gaza.

Police have arrested hundreds of Palestine Action supporters in recent weeks under anti-terrorism legislation, including over 500 in just one day last month, many of them over the age of 60.

On Saturday, hundreds of demonstrators gathered near parliament in central London to protest against the ban on Saturday, with many holding up signs that said: “I oppose genocide. I support Palestine Action.”

London’s Metropolitan Police said officers had begun arresting those expressing support for Palestine Action. Police did not say how many arrests were made but a Reuters witness said dozens of people were detained.

Palestine Action’s ban, or proscription, puts the group alongside al-Qaeda and ISIS and makes it a crime to support or belong to the organisation, punishable by up to 14 years in prison.

“I can be unequivocal, if you show support for Palestine Action — an offence under the Terrorism Act – you will be arrested,” Met Deputy Assistant Commissioner Ade Adelekan said on Friday.

“We have the officer numbers, custody capacity and all other resources to process as many people as is required.”

Human rights groups have criticised Britain’s decision to ban the group as disproportionate and say it limits the freedom of expression of peaceful protesters.

The government has accused Palestine Action of causing millions of pounds worth of criminal damage and says the ban does not prevent other pro-Palestinian protests.