British police arrested Swedish activist Greta Thunberg on Tuesday during a pro-Palestinian protest in London, according to UK-based campaign group Defend Our Juries.
The group said Thunberg was arrested under the Terrorism Act after holding a sign stating support for prisoners linked to Palestine Action, an organization that the British government has proscribed as a terrorist group.
A spokesperson for the City of London Police said two people had earlier been arrested for throwing red paint on a building.
"A little while later, a 22-year-old woman also attended the scene," the police spokesperson said in statement. "She has been arrested for displaying an item [in this case a placard] in support of a proscribed organisation [in this case Palestine Action] contrary to Section 13 of the Terrorism Act 2000."
Palestinian activist group risoners for Palestine said Thunberg had been holding a sign reading "I support the Palestine Action prisoners. I oppose genocide."
A number of experts, including those commissioned by a UN body, have said that Israel's offensive in Gaza amounts to genocide. Israel vigorously denies the claim.
Thunberg has spoken out previously about the hunger strikers, who are in prison awaiting trial after showing support for Palestine Action.
Defend Our Juries said the Tuesday protest targeted a building used by an insurance firm that, according to the group, provides services to the British arm of Israeli defense company Elbit Systems.
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The group says it exists to highlight the undermining of the legal process for people acting on their conscience. It has, in particular, supported the actions of Just Stop Oil and Insulate Britain climate campaigners and those protesting against the proscription of Palestine Action.
In June 2025, the UK Home Office announced plans to designate Palestine Action as a proscribed terrorist organization after activists broke into an airforce base.
The group vandalized two Royal Air Force Airbus A330 MRTT refueling aircraft, spraying red paint into their engines and causing damage that the government said went beyond protest and posed a serious risk to national security.
The terrorist organization designation makes it a criminal offense to support the group, display its symbols, or express support for it in public.
The decision was controversial and has been challenged by civil liberties groups, who argue the group's actions constitute criminal damage rather than terrorism.