British police arrested Swedish activist Greta Thunberg in London on Tuesday at a pro-Palestinian protest, UK-based campaign group Prisoners for Palestine said.

It said Thunberg was arrested under the Terrorism Act for holding a sign that said “I support the Palestine Action prisoners. I oppose genocide”. The British government has proscribed Palestine Action as a terrorist group.

A City of London spokesperson said two other people had been arrested for throwing red paint at a building.

The spokesperson said a 22-year-old woman later attended the scene and was arrested for displaying a placard in support of a proscribed organization.

Prisoners for Palestine, which supports some detained activists who have gone on hunger strike, said the building had been targeted because it was used by an insurance firm which they said provided services to the British arm of Israeli defense firm Elbit Systems.

The insurance company did not immediately respond to a request for comment.

Thunberg, 22, became prominent after staging weekly climate protests in front of the Swedish parliament in 2018.

Last year, she was cleared of a public order offence in Britain as a judge ruled police had no power to arrest her and others at a protest in London the year before.

She was detained along with 478 people and expelled by Israel in October after joining an activist convoy of vessels, the Global Sumud Flotilla, that attempted to reach Gaza with aid supplies. Israel has consistently denied genocide allegations.

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(Reporting by Muvija M and Sam Tabahriti; Editing by Kate Holton and Alison Williams)