Greta Thunberg, Tracey Emin and Gary Lineker are among those to sign an open letter in support of Southbank Centre chair Misan Harriman, after what they call a “dishonest smear campaign” by media outlets that accused him of promoting Golders Green attack “conspiracies” and comparing Reform voters to Nazis.

Harriman, who has been chair of the Southbank Centre’s board of governors since 2021, was accused by the Telegraph of sharing a social media post that contained a “conspiracy” about the Golders Green attack because it questioned the amount of coverage given to the Muslim victim, Ishmail Hussein.

Critics of Harriman said the repost risked minimising the antisemitic nature of the attack. David Taylor, the Labour MP for Hemel Hempstead, said: “These posts are not only incredibly inappropriate for the chair of a charity board, but for anyone in the public eye.”

The Telegraph ran a story with the headline “Southbank Centre chief ‘compares Reform victory to Holocaust’” after the activist quoted Susan Sontag in a video giving his thoughts after Reform’s historic local election results.

He said: “She said when thinking about the Holocaust, 10% of people in any population are cruel no matter what, and 10% is merciful no matter what and the other – this is important – the other remaining 80% could be moved in either direction.

“It’s such a profound way to look at us. In the context of yesterday’s election result it is something which I think is really topical.”

Karen Pollock, the chief executive of Holocaust Educational Trust, also criticised Harriman, asking: “How on earth could yesterday’s election results ever be comparable to the Holocaust?”

Robert Jenrick, the Reform MP, said the post was “disgusting” and called for him to be removed from his position at the Southbank Centre. He wrote: “This crass moron should be nowhere near a taxpayer-funded organisation.”

Amid widespread coverage of Harriman’s comments, the letter reflects concern that public figures are being silenced for speaking out at a moment of heightened tension over antisemitism. But critics argue that Harriman’s role leading a major publicly funded institution makes the scrutiny not only legitimate but necessary.

“The purpose of the smear campaign, which we repeat, is entirely without foundation in fact, is to traduce and marginalise Misan,” reads the letter. “And it is intended to send a message to others that if they speak out, they will be subject to harassment and threats.”

More than 245 people signed the letter, including Riz Ahmed and David Oyelowo, which said that “trying to silence responsible critics of Israel by smearing them as antisemitic does not protect Britain’s Jewish community”.

Several Jewish cultural figures, including Pulitzer prize-winning Sontag biographer Benjamin Moser, actor Morgan Spector and the photographer Jillian Edelstein, signed the letter.

Harriman told the Guardian: “We have reached the point where truth itself is being crushed by the very institutions that are supposed to uphold it. I will never whisper about the oppressed. I stand with truth, I stand by my right to use my voice to help others.”

The letter comes after 53,000 people backed a campaign to lobby the press regulator Ipso about the coverage, which is more than double the number of people who complained about Jeremy Clarkson’s 2022 column where he said he wanted the Duchess of Sussex “paraded naked through the streets of every town in Britain”.

A separate letter from parliamentarians has been sent to the culture secretary, Lisa Nandy, expressing concern over what the signatories describe as attempts to “mischaracterise” and “silence” Harriman.

The letter, seen by the Guardian, has been signed by the peers Sayeeda Warsi, Shaista Gohir and Meral Hussein-Ece; Labour MPs John McDonnell and Naz Shah; and Adrian Ramsay and Carla Denyer of the Greens. The signatories argue that questioning disparities in media reporting “should never have been a cause for concern” and warn against a “rising tendency to pressure institutions and public bodies to distance themselves from individuals who engage in legitimate public discourse”.

They condemn what they describe as a “smear campaign” against Harriman “seemingly aimed at whipping up a furore to engineer an ever-growing environment of cancel culture”.

The parliamentarians wrote that “at a time of heightened communal tensions, it is essential that all victims of violence and hatred are afforded equal visibility, dignity, and compassion in public discourse”. They added that efforts to delegitimise people speaking about anti-Muslim hatred risked “deepening division rather than fostering social cohesion”.

Harriman has built a large online following after emerging as a photographer during the Black Lives Matter protests and is the subject of a forthcoming documentary about his work made by Bafta-winning director Andy Mundy-Castle.

He was chosen by the Duke and Duchess of Sussex to shoot their portrait, while Harriman has worked with children who have fled Gaza, giving them equipment to shoot their own images. He is also an ambassador for Save the Children.

A spokesperson from the Southbank Centre said the institution was an “inclusive and welcoming place for everyone including our artists, audience and all colleagues.

“The Southbank Centre condemns all forms of antisemitism, hatred and discrimination. All Southbank Centre board members, including the chair, have the right to exercise their freedom of expression within the law. The personal views of individual members of our board do not represent the views of the Southbank Centre and in no way affect our programming nor the welcome that we extend to all.”

Additional reporting by Aamna Mohdin