LONDON — The picture was just like many of the other Facebook posts criticizing Britain’s decision to leave the European Union: a fake commemorative stamp showing a person preparing to shoot himself in the foot.

But on Tuesday, Facebook revealed that the unremarkable post was anything but. It originated from an Iranian-backed group aimed at Britain, in what the company said was the first known instance of a foreign influence campaign aimed at people outside the United States.

Facebook has spent the past two years trying to block foreign propaganda in the United States. But its disclosure of hundreds of fake accounts and pages, including the one tied to the Iranian-backed group, revealed that the foreign manipulation of elections through Facebook extends across the globe. Tactics used by Russia-linked groups ahead of the 2016 presidential election are being applied in Britain, the Middle East and Latin America.

Europe, where Facebook has more users than in the United States, is particularly worried. The company’s announcement exacerbated concerns that the region will be a regular target of foreign propaganda efforts — including ahead of next year’s European Parliament elections, which will help set the policy direction in Brussels for the next five years.