Open Society Foundations has hit out at Facebook as “reprehensible” after a PR agency hired by the social networking site reportedly tried to discredit critics by claiming they were agents of George Soros.

Patrick Gaspard, the president of OSF, said Facebook’s methods “threaten the very values underpinning our democracy” in a letter addressed to Sheryl Sandberg, Facebook’s chief operating officer and the executive responsible for the company’s political operations over last few years. OSF was founded by Soros in 1993 to fund civil rights groups around the world.

The New York Times reported on Wednesday that Facebook’s lobbying operation in Washington DC had attempted to distract from growing pressure over Russian interference in the 2016 presidential election by hiring a political consultancy to target opponents.

The consultancy, Definers Public Affairs, published dozens of negative articles about other tech companies on a “news” website that the PR firm ran in-house.

Definers also attempted to cast Soros, a long-standing target of antisemitic conspiracy theories on the far right, as the driving force behind Facebook critics. As late as this summer, the New York Times reported, the firm was circulating documents that put Soros as “the unacknowledged force behind what appeared to be a broad anti-Facebook movement”.

At the same time, Facebook lobbied “a Jewish civil rights group to cast some criticism of the company as antisemitic”, the newspaper reports.

In his letter to Sandberg, Gaspard noted that “there is a concerted rightwing effort the world over to demonise Mr Soros and his foundations, which I lead – an effort which has contributed to death threats and the delivery of a pipe bomb to Mr Soros’s home. You are no doubt also aware that much of this hateful and blatantly false and antisemitic information is spread via Facebook.

“The notion that your company, at your direction, actively engaged in the same behavior to try to discredit people exercising their first amendment rights to protest Facebook’s role in disseminating vile propaganda is frankly astonishing to me.

“These efforts appear to have been part of a deliberate strategy to distract from the very real accountability problems your company continues to grapple with. This is reprehensible, and an offense to the core values Open Society seeks to advance. But at bottom, this is not about George Soros or the foundations. Your methods threaten the very values underpinning our democracy.”

On Thursday morning, following the revelation of its relationship, Facebook fired Definers, the New York Times reported. In a statement, the company said: “It is wrong to suggest that we have ever asked Definers to pay for or write articles on Facebook’s behalf, or communicate anything untrue. The relationship with Facebook was well known by the media — not least because they have on several occasions sent out invitations to hundreds of journalists about important press calls on our behalf.”

Tim Miller (@Timodc) On a personal note I'm really blown up by the accusations. Im disgusted by the rise of anti-semitism including people who have falsely targeted Soros. It's deeply deeply personal. I've continuously fought the alt-right & others who spread racist lies & hate & will keep doing so

Tim Miller, Definers’ Silicon Valley lead and a former Jeb Bush spokesperson, wrote on Twitter: “Definers shared a narrow document about an anti-Facebook group’s funding. It was entirely factual, as Open Markets organizers have acknowledged they get funding from Soros. I have defended Soros from smears and conspiracies that weren’t based in fact.

“On a personal note I’m really blown up by the accusations. I’m disgusted by the rise of antisemitism including people who have falsely targeted Soros. It’s deeply deeply personal. I’ve continuously fought the alt-right and others who spread racist lies and hate and will keep doing so.”