A group called Youth Demand posted video showing a man in boots step into Sunak's pond, where he pretended to defecate.
The group said in a statement chock-full of a crude four-letter word for human waste that it was a “parting gift” to the prime minister. It said the stool used in the stunt was made of latex so it could be retrieved and prevent environmental damage.
Sunak was in London at the time for the state visit by the Japanese emperor and empress.
The incident comes just over a week before the U.K.’s general election that will determine if Sunak remains in power. Polls and pundits have predicted the Labour Party to take control after 14 years of Conservative rule.
The police officer who confronted the group asked the man identified by the group as “Oliver” what his intentions were, according to video of the incident.
“I think our intentions are carried out," he replied.
Youth Demand said it is calling for for a two-way arms embargo on Israel and for the next U.K. government to revoke oil and gas licenses granted since 2021.
The group said the four detained included a press photographer.
Sunak had condemned the group earlier this year when it hung a banner on the home of Labour leader Keir Starmer, saying “Stop the killing,” in reference to Israel's war with Hamas militants.
In August, four Greenpeace protesters were charged with criminal damage after climbing on Sunak's home while he was away and draping it in black fabric to protest his plan to expand oil and gas drilling in the North Sea.