The two-week festival gets underway with a gala screening of the stop-motion animation film, which is an adaptation of British author Roald Dahl's 1970 children's novel.
"There's a level of pride on film here that is really fun," Clooney said.
"It's a really great place to bring a movie and find out if it's going to hold up, and last not just for an opening weekend but whether it's going to work or not."
American actor Murray, who voices the character of Badger, felt the film would not have been made in the United States because they do not have the same capacity for high-quality stop-motion movies.
"Festivals are fun and you go with your movie to support your movie but this film couldn't have been made anywhere in the world but London, I don't think.
"We can put a man on the moon but we would not make this movie.
"There was more talent in that one little factory than I've ever been closeted with. I've never been with so many talented people in one place.
"They do things here with sets and design and models that America doesn't dream about."
The 53rd annual London Film Festival, which runs from Wednesday to October 29, will screen some 191 feature films and 113 shorts, including 15 world, 23 European and 146 British premieres.
London differs from many other European film festivals in focusing on bringing world movies to cinema-goers.
Red-carpet glamour mixes with innovation and experimentation, and many of the films would not otherwise get a British screening.
The festival's artistic director Sandra Hebron said the 2009 line-up was diverse and vibrant.
"I'm delighted that we have such a strong and varied programme this year, presenting new work from some of the world's most renowned directors alongside films from exciting new talents, and showcasing creativity and imagination from around the world.
"Film-makers, industry and media guests and our public audiences will join us in London for what promises to be an exciting, stimulating and enjoyable two weeks."
The festival will host a free outdoor screening in Trafalgar Square, while more than 20 archive short films will be shown with live piano accompaniment.
Source: AFP European Edition