Other premieres are Parkland, Peter Landesman's re-creation of John F. Kennedy's assassination and David Gordon Green's brutal Joe with Nicholas Cage as a violent ex-con who teams up with a homeless teen.
"This festival draws its strength from the risks it takes," this year's jury president, Oscar-winning Italian director Bernardo Bertolucci, said at a canalside cocktail party on the eve of the opening ceremony. Gondoliers could be seen shipping silver screen stars to Venice's Lido island where the 70th edition of the festival will run to September 7th, accompanied by a plethora of luxury yacht parties and beach soirees. Paparazzi were doused in sea spray as speed boats whipped past the shore of the Lido and starlets lounged in the Italian sun.
Alongside Clooney, Bullock and Cage, red carpet stars will include Scarlett Johansson, Matt Damon, Zac Efron and South Korea's Kim Ki-duk whose grim morality tale Pieta won the Golden Lion prize last year.
Twenty films are up for the Lion this year. British and American flicks dominate, among the most harrowing will be James Franco's Child of God, an adaptation of a Cormac McCarthy novel. The squeamish will also be tested by Jonathan Glazer's Under the Skin, in which Johansson stars as an alien who hunts down and devours unwitting hikers!
Monty Python star Terry Gilliam's drama The Zero Theorem is unlikely to lift the mood, with its bleak tale of solitude and madness centred around a race to decode a mathematical formula to discover whether life has any meaning.
Japanese animation master Hayao Miyazaki is also in the running with The Wind Rises, a World War II story adapted from Miyazaki's own manga.