Twenty films were vying for the Golden Lion as the world's oldest film festival opened in Venice on Wednesday. Actors Scarlett Johansson and Matt Damon joined Clooney and Bullock on the red carpet at Venice's Lido island.

"Gravity", directed by Mexico's Alfonso Cuaron and filmed in 3D, stars Clooney and Bullock as astronauts who are cast adrift in space after their shuttle is hit by debris.

Bullock plays Dr. Ryan Stone, a medical engineer on her first mission, who relies on veteran astronaut Matt Kowalsky (Clooney) to hold on to her sanity and try to survive despite her rapidly dropping oxygen levels.

At a news conference, Bullock and Clooney said the filming of "Gravity" had been one of the most demanding events in their careers.

"Physically and mentally it was the craziest, most bizarre, challenging thing. But you find what you're made of," Bullock said.

Cuaron's work is just one of several tense tales on the bill at the festival. Festival director Alberto Barbera said organisers watched 3,500 films before deciding on their final pick of 53 to be shown.

"You will be disappointed in some cases and satisfied in others. Don't expect a rollercoaster, but we have taken some calculated risks," he said.

Barbera said there was a “dark and violent reality” to the selected films, with filmmakers “not giving any sign of optimism.”

The festival's 10-member jury includes of Oscar winner Bernardo Bertolucci, Carrie Fisher of Star Wars fame and German actress Martina Gedeck ("The Lives of Others").

Reluctant juror wants to 'be surprised'

Bertolucci - director of 1972's "Last Tango in Paris" - said he had to be convinced to take part.

"[Barbera] made me face up to certain responsibilities that I am supposed to have before cinema and before those who are starting making cinema ... so here I am," he said.

"Unpredictable films" were highest in his esteem, he revealed. "What I want the most is to be surprised," he said.

The first three of the twenty films in the running for the Golden Lion will be shown on Thursday: "Via Castellana Bandiera" of Italy, Australia's "Tracks" and Germany's "Die Frau Des Polizisten" ("The Wife of the Policeman").

ph/ipj(dpa, AFP, AP)