London police yesterday arrested Swedish activist Greta Thunberg at a demonstration in support of pro-Palestinian hunger strikers, Palestinian activist groups said.

Thunberg's arrest makes her the highest profile person to be detained by police since the government banned the Palestine Action group under anti-terror laws.

"Greta Thunberg was arrested under the Terrorism Act at the Prisoners for Palestine lock-on protest," Prisoners for Palestine said in a statement.

Thunberg was holding a sign reading "I support the Palestine Action prisoners. I oppose genocide", it added. The 22-year-old was among hundreds of people who boarded a flotilla in October that tried to break through the Israeli blockade of Gaza.

City of London Police said officers had arrested three people on suspicion of criminal damage at a building in London's financial quarter.

It said the arrests came after "hammers and red paint were used to damage a building".

"A man and a woman have been arrested on suspicion of criminal damage. They glued themselves nearby and specialist officers are working to release them, and bring them into police custody," a statement said.

"A little while later, a 22-year-old woman also attended the scene. 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," it added.