London, UK - London police on Tuesday 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.

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Thunberg was holding a sign reading "I support the Palestine Action prisoners. I oppose genocide", it added, referring to Israel's assault on Gaza that has been deemed genocidal by countless experts and rights groups.

The 22-year-old was among hundreds of people who boarded a flotilla in October that tried to break through the illegal 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.

The government outlawed Palestine Action in July after activists broke into an air force base and caused an estimated £7 million ($9.3 million) of damage.

Some of the eight detainees who have been on hunger strike have been charged over that incident.

The group, aged between 20 and 31, are facing trials relating to break-ins or criminal damage by Palestine Action.

They are on hunger strike to protest their treatment and call for their release on bail.

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Asked about it in parliament last week, Prime Minister Keir Starmer said "rules and procedures" were being followed.

Two of the protesters began their hunger strikes in early November, according to their supporters, with others joining in the following weeks.

The government's Palestine Action ban – which makes being a member of the group or supporting it a serious criminal offense punishable by up to 14 years in prison – has resulted in at least 2,300 arrests of demonstrators, according to protest organisers Defend Our Juries.

According to London's Met Police in late November, so far 254 out of the more than 2,000 arrested have been charged with a lesser offense which carries a sentence of up to six months.