Two Hollywood legends made their comeback at the Cannes Film Festival on Sunday, as Kevin Costner launched his sprawling self-funded Western and Demi Moore returned in a gore-filled body horror flick.

The star-studded premieres arrived midway through the world's most famous movie festival, where "Emilia Perez" -- a musical about a transgender drug lord, starring Selena Gomez -- is currently the talk of the town.

Epic Western "Horizon, an American Saga" is a passion project for Costner, who told AFP he had started working on the screenplay in 1988.

Despite having enormous success with Westerns, including the Oscar-winning "Dances with Wolves", "Open Range" and television's "Yellowstone", he could not find a studio willing to fund his script.

"But I loved it and so I decided I would write four, which is very American of me -- insane," said Costner, who bankrolled the project himself.

The first film premiered at Cannes on Sunday in a special screening outside the main Palme d'Or competition. A second film has also been shot, and both will launch globally in theatres this summer.

"Horizon" follows multiple characters and storylines on the violent frontier, as Europeans establish settlements on Native American land.

Costner stars along with Sienna Miller and Sam Worthington.

Critics were underwhelmed, with the Hollywood Reporter dubbing it a "clumsy slog."

- Moore wows -

Another veteran US star, Demi Moore, took centre stage in "The Substance", a horror film that tackles the immense pressures society places on women to maintain bodily perfection as they age.

Moore starred in several hit movies in the 1990s, including "Ghost", "A Few Good Men", "Indecent Proposal" and "G.I. Jane".

This year's Cannes is a remarkable return to the red carpet, after years in which Moore has made headlines more for her marriages to Bruce Willis and Ashton Kutcher than her acting.

The ultra-graphic film drew gasps and raucous ovations throughout its screening, and also fared much better in reviews.

Deadline called the movie "the year's smartest, goriest horror breakout", while Variety singled out Moore as "nothing short of fearless".

"The Substance", directed by Coralie Fargeat, is among 22 films competing for the Cannes festival's top prize, the Palme d'Or.

Also on Sunday, Ben Whishaw drew rave reviews for his portrayal of a radical Soviet poet in "Limonov: The Ballad."

The Guardian said the British actor was "glorious" as a dissident writer who lived homeless in New York before returning to Russia and fronting a fascist group.

Indiewire hailed a "career-crowning" performance, which puts Whishaw among the favourites for the festival's acting prize.

Other high-profile entries that have played so far include Francis Ford Coppola's divisive epic "Megalopolis," Andrea Arnold's much-praised childhood saga "Bird" and Paul Schrader's widely panned "Oh, Canada".

- Audiard's audience -

The early frontrunner appears to be "Emilia Perez" from French auteur Jacques Audiard, who received a hero's welcome from the world's media as he hosted a press conference Sunday, a day after its world premiere.

Karla Sofia Gascon, a Spanish trans woman, stars as a Mexican drug baron who longs to change gender and escape the narco life.

"We're normal people who can have the careers they want," said Gascon.

Zoe Saldana plays a lawyer who must arrange the operation, which is kept secret from everyone including the narco boss's wife, portrayed by pop star-turned-actor Gomez.

The characters regularly break out into song, with lyrics tackling everything from plastic surgery to Mexico's struggles with corruption and warring drug gangs.

Audiard said he had originally envisioned the film as an opera.

The "crumbling of democracy" in Mexico, a country he finds to be "schizophrenic", provided the perfect setting for "a tragedy", said the director.

Outside of the Palme d'Or competition, Cate Blanchett launched "Rumours", which follows a group of world leaders who meet for a G7 summit but end up getting lost in a forest, with debauchery ensuing.

"I think if you try to make sense of this movie, you'll feel like you're losing your mind," joked Blanchett.

The Palme d'Or winner will be unveiled on Saturday, the festival's closing day.