By Silvia Aloisi

ROME (Reuters) - "Juno", the story of an American teenager faced with an unplanned pregnancy, won the top prize at the Rome film festival on Saturday.

The film is directed by 30-year-old Canadian-born Jason Reitman, whose 2005 comedy "Thank you for Smoking" scooped a string of awards and was nominated for two Golden Globes.

"It's terrifying to bring your film to another culture," Reitman said, calling his film "feminine".

"They say you don't really know another culture until you can make them laugh, so to hear your laughter it warms my heart and makes me feel that perhaps we are closer than we all think," he said.

Critics praised the performance by actress Ellen Page as Juno MacGuff, the quick witted young woman who falls pregnant at her first sexual experience.

Suddenly plunged into adulthood, she sets out to find a suitable set of parents to adopt her unborn child.

The film, one of 14 titles in competition, also casts Jennifer Garner as the affluent, prospective adoptive mother whose marriage is not as idyllic as it seems.

The jury, made up of 50 ordinary movie-goers, also awarded a special prize to Iranian director Abolfazl Jalili for his film on a renowned Persian poet, "Hafez".

Outside the competition, Sean Penn won a sponsors' award for "Into the Wild", the real-life story of a 24-year-old adventurer and his two-year, journey of self-discovery from South Dakota to Alaska.

The award ceremony wrapped up the second season of the 10-day movie marathon that was launched last year by Mayor Walter Veltroni, irking organisers of the venerable Venice film festival who see it as an unwelcome rival.

The "Festa del Cinema" drew to Rome a string of Hollywood stars and veteran film-makers including Francis Ford Coppola, who picked the festival to present his first film in 10 years, "Youth Without Youth".

But some critics said the Rome event, whose stated aim is to be a showcase of quality films for wide audiences and not just the industry or the press, lacked a clear identity. The competition was generally regarded as lacklustre, with the A-list of actors and directors kept outside the contest.

Some commentators quipped that the most eye-catching event of the festival had been a vandal turning the waters of the famed Trevi Fountain blood-red with a bottle of dye, partly in protest at the money spent to roll out the red carpet.

(Additional reporting by Cristiano Corvino and Fabio Severo)