London ordered the removal on Wednesday of adverts stating "Israel is a racist endeavour," which were featured at bus stops around London, although it is not clear who erected them. Police have opened an investigation following the posters' appearances in at least four locations, including stops in Westminster, Waterloo and Bloomsbury.

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The posters appeared a day after the U.K. Labour Party adopted the International Holocaust Remembrance Alliance's definition in an attempt to defuse a row deepening rifts on its leader, Jeremy Corbyn.

A section of the definition states that "denying the Jewish people their right to self-determination, e.g., by claiming that the existence of a State of Israel is a racist endeavor."

A spokesperson for London mayor Sadiq Khan said: "These offensive adverts are not authorized and are acts of vandalism which Transport for London and its advertising partner takes extremely seriously. They have instructed their contractors to remove any posters found on their network immediately."

Transport for London said the posters were “absolutely not authorized” and would be removed from the network “immediately."

The group London Palestine Action tweeted photos in support of the posters around the city, though others quickly condemned them.

In a tweet by the group, the status read: "Rights denied based entirely on ethnicity. Israel is a #racistendeavour."

"This is ugly and intimidating. As a British Jew I find it quite frightening. The context is what has become a hostile and damaging debate over antisemitism," tweeted Adam Wagner, a human rights barrister.

A tweet by another individual read: "I had to do a double take when I saw *this* on Westminster Bridge."