London's Metropolitan Police arrested 41 people on Saturday who showed support for the Palestine Action movement at a demonstration outside Parliament.
In a statement on social media, London police said they had "arrested 41 people for showing support for a proscribed organization, and one person was arrested for assault."
Police arrested 29 people after a similar demonstration in London last week.
Police also made arrests at a demonstration in support of Palestine Action in Manchester. Other protests took place in Cardiff and Northern Ireland.
British lawmakers banned the movement under so-called anti-terrorism legislation earlier this month, after some of its members stormed a Royal Air Force base and damaged two aircraft in protest against Britain's support for Israel.
Prior to today's arrests in London, approximately 50 protesters gathered outside Parliament carrying signs reading "I am against genocide. I support Palestine Action" near a statue of the late South African president Nelson Mandela.
The International Court of Justice in The Hague is considering a case filed by South Africa accusing Israel of committing genocide against Palestinians in the war on Gaza.
The British government's decision to designate Palestine Action as a terrorist group places it in the same category as Hamas, al-Qaeda, and the Islamic State. Membership in the group is now punishable by up to 14 years in prison.
Opponents of the ban say the use of anti-terrorism laws is inappropriate against a group focused on civil disobedience.
Palestine Action has generally targeted Israeli and Israeli-linked companies in Britain, such as Israeli defense contractor Elbit Systems, often spraying red paint on facades, blocking entrances, or damaging equipment.
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Britain arrests dozens of supporters of a pro-Palestinian movement.