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

Thunberg was arrested under the Terrorism Act for holding a sign that said she supported prisoners linked to Palestine Action, an organisation which the British government has proscribed as a terrorist group, Defend Our Juries said.

A City of London spokesperson said two people had 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."

Defend Our Juries said it targeted the building because it was used by an insurance firm which they said provided services to the British arm of Israeli defence firm Elbit Systems.