US actor George Clooney (R) poses with actress Sandra Bullock during a photocall at the 70th Venice Film Festival in Venice August 28, 2013.[Photo/Agencies]

The Venice Film Festival got off to an explosive start on Wednesday with the premiere of 3D space movie "Gravity", teaming George Clooney and Sandra Bullock as astronauts imperiled by a botched Russian missile shot.

Twenty films, including five from the United States and three British ones, are in competition for the Golden Lion trophy that will be awarded at the end of the 70th edition of the world's oldest film festival, on September 7.

"I hate space," Bullock's astronaut character Ryan Stone says midway through what essentially is a Clooney-Bullock two hander in which their shuttle is destroyed by an avalanche of space debris triggered by a Russian missile strike on a spy satellite gone horribly wrong.

She does not learn to love space, but Stone, a scientist who has been grieving over the death of her four-year-old daughter, has a spiritual rebirth, thanks in part to the ebullient resilience of Clooney's country-music-loving Matt Kowalsky and to her discovering her own instinct to survive.

At the post-film news conference, Bullock and Clooney said the film, directed by Mexico's Alfonso Cuaron, had been one of the most demanding in which they had ever appeared.

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

In possibly the strongest female space acting performance since Sigourney Weaver killed the female reptile in "Alien", Bullock overcomes waves of debris that destroy pretty much everything in orbit as Stone and Kowalsky embark on a roller-coaster ride from hell to get back to earth.

There is no sound in space, the movie says in its opening titles, but the pounding soundtrack, and the 3D explosions on board one disintegrating space vehicle after another strongly suggest otherwise. There is more suspense in the movie's one and half hours than any of the online trailers have suggested.

David Rooney, critic for the Hollywood Reporter, told Reuters of "Gravity", which is not in competition: "You sit there watching completely immersed in this atmosphere and puzzling over how they achieved all this."

Clooney and Bullock open 70th Venice Film Festival	Jury members pose during photocall