Climate activist Greta Thunberg being detained by City of London Police on December 23, 2025. (Photo by PA media)

Climate activist Greta Thunberg has been arrested in central London during a demonstration in support of imprisoned Palestine Action activists on hunger strike, as pressure mounts on the UK government over the rapidly deteriorating health of the detainees.

Thunberg was detained on Tuesday while holding a placard reading, "I support the Palestine Action prisoners. I oppose genocide," at a protest organised by Prisoners for Palestine outside the London offices of Aspen Insurance.

Activists say the company provides services to Israeli-linked military firm Elbit Systems. Two other protesters were arrested after spraying red paint on the building`s entrance.

Police said officers responded after "hammers and red paint were used to damage a building on Fenchurch Street," adding that a man and a woman were arrested on suspicion of criminal damage.

Thunberg was later arrested for displaying a placard "in support of a proscribed organization," police said, referring to Palestine Action, which the UK government banned in July under the "Terrorism Act."

Thunberg`s arrest comes amid a deepening crisis involving Palestine Action prisoners who have been on hunger strike since November to protest their continued detention and the banning of the group. At least seven prisoners have been taken to the hospital since the hunger strike began.

The prisoners have now initiated legal proceedings against the government of Prime Minister Keir Starmer, accusing it of abandoning its own prison safety policy framework.

A legal firm representing the activists has submitted a pre-action letter to Justice Secretary David Lammy after several detainees were hospitalized due to rapidly deteriorating health.

The letter demands a response within 24 hours, warning that "the risk of their dying increases every day," and calls for an urgent meeting to address the prisoners` condition.

Lawyers for the detainees had previously sought urgent intervention from Lammy, who is also the deputy prime minister, but his response failed to address their concerns.

Swedish activist Greta Thunberg was arrested by UK police under the Terrorism Act at the Prisoners for Palestine lock-on protest outside the offices of Aspen Insurance in London, which insures the Israeli weapons firm Elbit Systems. pic.twitter.com/VIvqCGjrML

— Al Jazeera English (@AJEnglish) December 23, 2025

Medical professionals have also issued stark warnings. In a letter signed by more than 800 doctors, emergency physician Dr James Smith said the hunger strikers face a "very high risk of serious complications, including organ failure, irreversible neurological damage, cardiac arrhythmias, and death."

Among them is Kamran Ahmed, 28, who has been on hunger strike for 42 days. Speaking by telephone from prison, he said the protest was worth the risk.

"Every day I`m scared that potentially I might die," Ahmed told The Times on Sunday. "But this is bigger than me."

The prisoners, who are held on remand without trial, face charges linked to alleged break-ins or criminal damage carried out before Palestine Action was banned.

They are demanding immediate bail, lifting of the ban, closure of arms firms linked to Israel, and restoration of communication and legal rights in custody.

Since the implementation of the ban, more than 2,000 people across the UK have been arrested for holding placards with the words, "I oppose genocide, I support Palestine Action."

Campaigners say the arrests and prosecutions are politically motivated and aimed at silencing opposition to the UK government`s support for Israel.