A series of "counter-propaganda" posters popping up in London bus stops have accused the Met Police of racism and corruption.
The campaign criticises the force's stance on drugs, its involvement in the shooting of Mark Duggan, and its treatment of racial minorities.
One poster, describing the Met Police as "still really racist" is taking up advertising space outside New Scotland Yard.
Strike! Magazine denies putting the posters up, but has shared information on "interacting" with advertising spaces on its Twitter account.
A spokesperson for the magazine told the Huffington Post: "The response has been overwhelmingly positive. We're really happy they're going up."
They also said that people have been stopped from taking pictures at the sites or had their phones confiscated by police.
In a Facebook post, Strike! said: "If only the Met put this much effort into investigating their own crimes and corruption."
One image that appeared was in response to a £33,000 campaign by the Met regarding cannabis use in Lewisham.
Strike! told the Huffington Post that they believed UK drug laws were racist, because they focus on the consumers of drugs rather than the effects caused.
The signs are occupying advertising space owned by Clear Channel, while a separate propaganda campaign by Strike! has been spotted on London Underground trains.
Clear Channel said: "On Sunday 14th December, we received information that some of our sites had been accessed and the advertising had been replaced by posters from an activist group.
"Our Operations team are attending the sites throughout the night and are re-posting the advertising, in line with our standard practice."
The Metropolitan Police refused to comment on the issue.