In a normal Oscars week, Jeremy Zimmer would expect to be wearing his best tux, rubbing shoulders with the biggest stars in Hollywood and grabbing canapés with one hand while holding a glass of champagne in the other. This year he expects to be holding nothing more glamorous than a placard.

“Don’t get me wrong, I like lobster rolls and handfuls of lamb chops as much as anyone, and more than most,” Zimmer – the head of United Talent Agency, which represents the cream of Hollywood, from Angelina Jolie to Will Ferrell – told his staff in an email. “However, this seems like a moment for us, at UTA, to speak up and put our money where our mouth is.”

The moment was the realisation that one of UTA’s clients, the Iranian director Asghar Farhadi, who won the best foreign language Oscar in 2012 and has been nominated in the same category at next Sunday’s Academy Awards, would not be able to attend the ceremony because of President Trump’s proposed ban on visitors from seven majority-Muslim countries.

In a gesture of solidarity with Farhadi, Zimmer announced that, instead of holding its annual Oscars party – a highlight of the glitziest week of Hollywood’s year – UTA would hold a rally in Los Angeles on Friday and donate the $250,000 it would have spent to the American Civil Liberties Union and the International Rescue Committee. With a client list that includes Gwyneth Paltrow, Don Cheadle and Bryan Cranston, it could be a very glitzy refugee rally.

Next weekend’s event will be the most public showing to date of a growing activism in Hollywood and across the broader cultural sector – a trend set by Meryl Streep in a speech attacking Trump at the Golden Globes in January and given urgency by measures such as the travel ban and the anticipated axing of the National Endowment for the Arts, the main source of funding for the arts in the US.

At the Oscar nominees luncheon earlier this month, the academy’s president, Cheryl Boone Isaacs, declared that the current political climate presented artists with a set of challenges not seen for more than half a century.

“There is a struggle globally today over artistic freedom that feels more urgent than at any time since the 1950s,” she said. “Art has no borders. Art has no language, and doesn’t belong to a single faith. No. The power of art is that it transcends all these things and strong societies don’t censor art; they celebrate it.”

Clients at other Hollywood agencies have challenged them to follow the example set by UTA: WME IMG, another talent behemoth, announced this month that it was forming a political action committee. “This company’s greatest asset is the diversity of our backgrounds and beliefs,” read a statement put out by the agency – whose clients included the former host of Celebrity Apprentice turned president, Donald J Trump. “Please know that we will do everything in our power to support and protect this diversity now and in the months and years ahead.”

The sentiment has found an echo in Britain, where Farhadi’s Oscar-nominated film, The Salesman, will receive its premiere on Oscar day at a free screening before an audience of up to 10,000 people in Trafalgar Square, as well as being screened in cinemas nationwide.

“I was incoherent with anger and terrified at the ramifications of the travel ban when it was announced, and I just thought, what can I do?” said film-maker Mark Donne, who, with producer Kate Wilson, came up with the idea for the screening.

Initially the plan was to stage it in London’s Grosvenor Square, by the US embassy. “I thought it would be quite lovely to have a screening in the spirit of peaceful civic protest, a sort of cultural communion,” Donne said.

He and Wilson gathered support from a range of stars, including Julie Christie, Keira Knightley and Lily Cole. When that plan foundered, Cole brought in Sadiq Khan, the mayor of London, who offered to stage what would be the film’s premiere in Trafalgar Square as part of his efforts to pitch the capital as a beacon of diversity and freedom.

“This unique premiere, in one of the world’s most famous public squares, will give us an opportunity to celebrate what makes us who we are – a city open to creative talent and all countries and communities,” Khan said on Saturday.

For Wilson, while the screening will be a celebration of London’s inclusiveness, “no one’s pretending there isn’t a subtext there.

Facebook Twitter Pinterest Jeremy Zimmer, head of talent agency UTA, has cancelled its annual Oscars party and will hold a protest rally instead. Photograph: Alberto E. Rodriguez/Getty Images for Chrysalis

“If you have a great love of America then it is very upsetting to see what is going on. It’s becoming a xenophobic, terrifying part of the world. I do think that people feel very strongly about this and don’t know what to do – I’m not a marcher – so one thing you can do is to have your voice heard by standing behind a creative individual.”

Farhadi, who will deliver a message by video from Tehran to London, also acknowledged the symbolism. “Screening The Salesman in Trafalgar Square has a great symbolic value for me. The gathering of the audience around The Salesman in this famous London square is a symbol of unity against the division and separation of people.”

Farhadi will be introduced by director Mike Leigh, who served with the Iranian on the jury at the Berlin film festival in 2012. “We must show solidarity with Asghar and his principles, and against divisiveness and hate,” Leigh said, describing Farhadi, who won the Oscar for best foreign film in 2012 for A Separation, as “one of the world’s greatest film-makers. For those of us who make movies about real life, real people and real issues, he is the master – a true inspiration to all of us.”

Mariella Frostrup, who will present the Trafalgar Square event, said it was important to take a stand against “the tsunami of total rubbish coming across the Atlantic”, adding: “It’s ironic that Farhadi can’t go to the Oscars because of Trump’s ludicrous proposition that barring people from seven countries will somehow make the world a safer place. It’s important that we make whatever small attempts we can to change the tide. It’s easy to laugh at it, but after that it becomes terrifying.”

While the ban has been suspended following a ruling by a federal court, the White House has said a new executive order will be signed in the coming days.

Announcing his decision to stay away from the awards, Farhadi said: “Hard-liners, despite their nationalities, political arguments and wars, regard and understand the world in very much the same way … To humiliate one nation with the pretext of guarding the security of another is not a new phenomenon in history and has always laid the groundwork for the creation of future division and enmity.”