The statue of Sir Winston Churchill in Parliament Square in Westminster was defaced overnight with "Zionist war criminal" and other graffiti.
A 38-year-old man has been arrested on suspicion of racially aggravated criminal damage and remains in custody.
Officers from the Metropolitan Police were alerted shortly after 4am on Friday.
Other phrases including "Stop the Genocide" and "Free Palestine" were sprayed in red paint on the bronze sculpture.
Further graffiti read "Never again is Now" and "Globalise the Intifada".
Dutch group Free the Filton 24 claimed responsibility for the action on Friday morning.
It posted a video on its Instagram account appearing to show a man dressed in red coveralls, with "I support Palestine Action" written on the back, painting the statue.
Free the Filton 24 defines itself as a group of "family and friends" of the 24 Palestine Action activists who were charged over a break-in at one of Israel-based defence firm Elbit's UK sites in 2024.
Olax Outis, who said he is Dutch and part of the group, has claimed to be the man on the statue.
In a statement on its Instagram account, Mr Outis said he defaced the statue "to draw attention to the horrible human rights violations happening in a country that's run by colonisers who refuse to listen to their people".
He added: "The current British Government should be dragged before the International Court of Justice in The Hague, and as a representative of The Hague, I'm here to hold them accountable."
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Explaining why he chose to target the Churchill statue, Mr Outis said: "To be blunt: if someone would ever be completely out of their mind enough to erect a statue of Keir Starmer or Yvette Cooper, I would happily demolish such an effigy.
"Churchill is but a symbol of the same political corruption."
The statue has been cordoned off and was being cleaned this morning.
A Greater London Authority spokesperson said: "We are appalled by this vandalism to the statue of Sir Winston Churchill and work is under way to remove the graffiti as quickly as possible."
Commenting on an image of the monument, which was shared on X, Dave Rich, director of policy for Community Security Trust, posted: "Free Palestine" and a Hamas red triangle, if you zoom in close enough. This extremism is never just a threat to Jews."
The Jewish Leadership Council said it was "disgusted" by the defacing of the statue.
It said on X: "In targeting the statue of a British hero who led this country in the fight against the Nazis, the perpetrator has found a perverse way to combine a hatred of Jews with a disdain for Britain."
Phil Rosenberg, president of the Board of Deputies of British Jews, wrote: "One of the greatest champions for liberty, who defeated the Nazis, defaced.
"Zionism, the national liberation movement of the Jewish people, inverted.
"Santayana's 'Those who cannot remember the past are condemned to repeat it', never truer."
The 12ft-tall statue on the north-east corner of the square, created by Ivor Roberts-Jones, was unveiled in 1973 by the former prime minister's wife, Lady Clementine Churchill.
It is one of 12 statues on or around Parliament Square, most of well-known statesmen such as Nelson Mandela and Abraham Lincoln.
The former prime minister's statue has been vandalised several times over the years, including during demonstrations.
Last December, both the Metropolitan Police and Greater Manchester Police announced anyone chanting "globalise the intifada" would face arrest.
The decision by the two police forces came in the wake of the Bondi Beach terror attack, and the terror attack at Heaton Park synagogue in Manchester on 2 October.
(c) Sky News 2026: Man, 38, arrested after Churchill statue defaced with 'Zionist war criminal'
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