By Agnieszka Flak

VENICE (Reuters) - Damien Chazelle's "La La Land", a musical comedy-drama about the joys and pains of pursuing one's dreams, opened the Venice film festival on Wednesday, kicking off days of screenings, parties and red carpet glamour.

The movie, starring Ryan Gosling and Emma Stone, tells the story of a struggling jazz pianist, Sebastian, who falls in love with Mia, an aspiring actress in modern day Los Angeles. Sebastian is trying to get people, young and old, to care about traditional jazz, while Mia hopes one of her many auditions will finally land her an acting gig.

"Young people have fallen into a lot of cynicism, are making fun of things and pointing out the flaws in everything and this movie is anything but that," Stone told journalists at a news conference. She said the movie reminded her of her own "humiliating auditions" when she first arrived in Los Angeles.

"This is what I hope young people will do: work hard to achieve their dreams."

The musical is one of 20 U.S. and international vying for the Golden Lion that will be awarded on Sept 10.

British director Sam Mendes, known for James Bond movies "Skyfall" and "Spectre" and who heads this year's jury, said he is looked forward to seeing each movie with as little knowledge as possible, adding he had no criteria for what made one excellent.

"I will know it when I see it," he said.

"La La Land" opens with a chorus music scene set during a morning traffic jam on a Los Angeles motorway, where drivers jamming away to tunes in their empty cars suddenly jump out of their vehicles and dance on the roofs, all to the beats of a big band.

Chazelle said the movie builds on all the clichés people may have about Los Angeles, including the "traffic, the terrible parties, the celebrity culture, the shallowness", but then shows that "there is something very poetic about the city that's just built by people with these unrealistic dreams".

The 31-year-old Chazelle is best known for writing and directing "Whiplash", which earned him five Academy Award nominations and won three Oscars.

He said the success of "Whiplash" - about a jazz drummer - had helped make "La La Land" happen, and its conclusion shows that dreams, including his own, can happen.

"I still pinch myself ... it was definitely a dream come true and a dream I've had for a long time," he said.

Chazelle's first foray into the musical genre was his first feature film, "Guy and Madeline on a Park Bench", he made when he was still at Harvard University.

"You take the magic of the old fashioned musical that we grew up loving but make it about real life today ... where reality doesn't always live up to the dream," he said.

The movie was well received by critics.

(Editing by Jeremy Gaunt)