Sharon Stone will attend a premiere of her new film Bobby

Artistic director Marco Mueller called the decision "a gamble".

"The successes scored at the box offices and at the Oscars by last year's Venice premieres pushed us to make this choice," he said

Twenty one films will compete for the Golden Lion, which was won last year by Ang Lee's Brokeback Mountain.

Sir Anthony Hopkins, Sharon Stone, Scarlett Johansson and Sir Michael Caine are among the stars attending premieres of their films in Venice, between 30 August and 9 September.

Competing films include Bobby, written by actor Emilio Estevez, about the final days leading up to Senator Robert Kennedy's assassination, and Alfonso Cuaron's sci-fi thriller Children of Men.

Brian De Palma's The Black Dahlia, also in competition, will open the 63rd festival.

Festival director Marco Mueller said Venice's ability to attract so many big-name stars and world premieres "demonstrates the affection for and the faith in the festival".

Diana drama

The lone British film in competition is The Queen, from director Stephen Frears. It stars Dame Helen Mirren as Queen Elizabeth II in the days following Princess Diana's death.

The film portrays the efforts of the Queen and Prime Minister Tony Blair to reach a compromise between private family grief and the public's demand for an overt display of mourning.

Ang Lee won the festival's top prize in 2005 for Brokeback Mountain

The winning film will be decided by an official jury headed by French actress Catherine Deneuve.

Films screening out of competition include Oliver Stone's World Trade Center and Manoel de Oliveira's Belle Toujours, a sequel to the French film classic Belle du Jour.

Other highlights include a special on Roberto Rossellini, marking the centenary of the Italian film-maker's birth, and a section dedicated to Soviet-era films called The Secret History of Russian Cinema.

"This year we have films from 27 countries against 18 for last year, including Chad, Malaysia and Indonesia," said Mr Mueller.

"It's great to have this kind of diversity."