Big-screen stars including Joaquin Phoenix, Ben Stiller and Kristen Stewart were among 1034 signatories to the message, which was published by The New York Times and made available via BlocktheMerger.com.
The intervention comes after Paramount Skydance, led by David Ellison, announced plans in February for a $US111 billion ($A157 billion) acquisition of Warner Bros. Discovery, headed by David Zaslav, following competition with Netflix.
The letter warns of the impact on jobs, competition and creative output across the industry.
It states: "We are deeply concerned by indications of support for this merger that prioritise the interests of a small group of powerful stakeholders over the broader public good. The integrity, independence, and diversity of our industry would be grievously compromised. Competition is essential for a healthy economy and a healthy democracy. So is thoughtful regulation and enforcement."
In response, Paramount said: "We hear and understand the concerns that some in our creative community have raised and respect the commitment to protecting and expanding creativity."
It added: "Importantly, as creators we know firsthand that this is also a moment when the industry has been facing significant disruption – and the need for strong, creative-first and well-capitalised companies that can continue to invest in storytelling has never been greater."
The company also said: "This transaction uniquely brings together complementary strengths to create a company that can greenlight more projects, back bold ideas, support talent across multiple stages of their careers, and bring stories to audiences at a truly global scale – while strengthening competition by ensuring multiple scaled players are investing in creative talent."
Other signatories include Adam McKay, Bryan Cranston, Cynthia Nixon, David Fincher, Denis Villeneuve, Glenn Close and Jane Fonda.
JJ Abrams, Jason Bateman, John Leguizamo, Lin-Manuel Miranda, Mark Ruffalo, Rosario Dawson and Tiffany Haddish also added their names.
Damon Lindelof, who is under an overall deal with Warner Bros Discovery's HBO, said on Instagram: "It's thousands and thousands of Grips and Gaffers. Drivers and Decorators. Builders and Boom operators. Camera teams and Caterers. And they're all about to get f****d.
"Hollywood mergers mean fewer movies and fewer TV shows and that means fewer jobs. When two storied back lots are owned by the same company, the outcome is intuitive – one becomes a ghost town.
"I'm scared. But I'm not a ghost. And a fight is already lost if it's never fought. So I signed. Proudly. I understand why many of my peers have not – trust me, I'm more of a puker than a stormer. But these boats are heading for the beach whether we like it or not."
19°C