London police on Tuesday arrested Swedish activist Greta Thunberg at a demonstration in support of pro-Palestinian hunger strikers, Palestinian campaign 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.
Prisoners for Palestine, which organized the protest, said in a statement that Thunberg was arrested under the UK Terrorism Act.
Thunberg, 22, was holding a sign reading: "I support the Palestine Action prisoners. I oppose genocide."
City of London Police said several people were arrested.
They did not directly name Thunberg, but said "a 22-year-old woman... has been arrested for displaying an item (in this case a placard) in support of a proscribed organization (in this case Palestine Action) contrary to Section 13 of the Terrorism Act 2000".
Police said another three people were arrested at the protest, at a building in London's financial quarter, on suspicion of criminal damage.
The three were detained after "hammers and red paint were used to damage a building" and they glued themselves to fixtures nearby, police said.
Prisoners for Palestine said its protest had targeted the offices of Aspen Insurance because the company provided services to Israeli-linked defense firm Elbit Systems UK.
- 'Political prisoners' -
Thunberg on Monday described the detained hunger strikers as "political prisoners" in a video posted on Instagram.
The British government in July outlawed Palestine Action 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 went on hunger strike had 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.
Their hunger strike is to protest their treatment and call for their release on bail.
The first two prisoners going on the hunger strike were on their 52nd day, Prisoners for Palestine said on Tuesday. The Guardian newspaper reported that three of the eight had ended their hunger strike.
Asked about it in parliament last week, Prime Minister Keir Starmer said "rules and procedures" were being followed.
The government's ban on Palestine Action -- which makes being a member of the group or supporting it a serious criminal offence punishable by up to 14 years in prison -- has resulted in at least 2,300 arrests of demonstrators, according to protest organizers 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 offence which carries a sentence of up to six months.
Thunberg has maintained a high profile in protests supporting Palestinians.
In October, she was among hundreds of people who boarded a flotilla that tried to break through the Israeli blockade of Gaza.
