George Clooney and Sandra Bullock sign autographs on the red carpe t Oscar-winning actors George Clooney and Sandra Bullock have helped launch the 70th Venice Film Festival with their 3D sci-fi thriller Gravity.
The Hollywood duo were joined by the film's Mexican director Alfonso Cuaron at Wednesday's world premiere.
Shot in 3D, Gravity tells of two astronauts who are cast into deep space when debris destroys their shuttle.
Nicolas Cage, Matt Damon, Zac Efron and Scarlett Johansson are among the other stars expected over the next 10 days.
Italian director Bernardo Bertolucci heads this year's competition jury.
Star Wars actress Carrie Fisher and British film-maker Andrea Arnold are among those helping him decide which of the 20 titles in contention should win this year's Golden Lion award.
They include Terry Gilliam's dark fantasy The Zero Theorem, in which Damon appears alongside double Oscar-winner Christoph Waltz, and Johansson's Scotland-set sci-fi tale Under the Skin.
Joe, starring Cage as a violent ex-convict, is also in the competition line-up, as is Parkland, a historical drama set on the day President John F Kennedy was assassinated in 1963.
High School Musical star Efron forms part of its large ensemble cast, playing the doctor who examined JFK's body at the Dallas hospital that gives Peter Landesman's film its name.
For the first time, two documentaries are up for the Golden Lion: Errol Morris's The Unknown Known: The Life and Times of Donald Rumsfeld, and Sacro GRA, a film about Rome's ring road.
British hopes rest on Gilliam's film, Jonathan Glazer's Under the Skin and Philomena - Stephen Frears' fact-based film about an Irish woman, played by Dame Judi Dench, trying to locate the son she gave up for adoption in the 1950s.
Bookmaker Paddy Power has ranked Philomena as second favourite to win the Golden Lion, behind Stray Dogs - the only Chinese language film in the competition.
"This year's festival favourites pack a powerful punch," said a spokesman. "We predict Stray Dogs however to have just a bit more bite than the rest of the pack."
Special awards will be presented this year to US director William Friedkin, Polish filmmaker Andrzej Wajda and the Italian writer-director Ettore Scola.
Clooney, Bullock and Cuaron attended a press conference for their film on Wednesday before attending its gala screening at the festival's Palazzo del Cinema.
Early reviews were fulsome in their praise, with the Hollywood Reporterdescribing it as a "jaw-dropping space thriller" boasting "breath-catching tension and startling surprise".
Variety's reviewer echoed those sentiments, saluting "a nervy experiment in blockbuster minimalism" that "offers in abundance the sort of eye-popping, screen-filling spectacle that demands to be viewed in a [movie] theatre".