Larry Clark, with stars of Bully, defended the film's violence
As the festival got under way on Thursday, the most talked about films were Bully, Y Tu Mama Tambien and Address Unknown - all containing graphic and disturbing scenes.
American movie Bully is a chilling story of teenage murder. Mexico's Y Tu Mama Tambien focuses on two teenagers' sexual initiation.
South Korea's Address Unknown, meanwhile, sees dogs savagely beaten to death.
It was not much to lift the spirits of the movie-goers at the 11-day event.
But as all three films are competing for the coveted Golden Lion award, few festival-goers could avoid at least one of them cropping up in conversation.
'Sham'
Of the three, Bully emerged as the most controversial. It is directed by Larry Clark and tells the real-life story of the murder of a 20-year-old by his teenage best friend.
Drugs, prostitution and rape all feature - but Clark rejected accusations that his movie was over the top.
"I tell it like it is. It is based on a true story, nothing was made up," Clark said. "We were dealing with reality so we wanted to make it real."
Alfonso Cuaron, the director of Y Tu Mama Tambien, was similarly defensive about the sex scences in his feature.
"The film talks about the search for identity by two adolescents,'' Cuaron said.
"I hope that the film will be seen as it is. Sex is not the story, but is part of the story.'
Basically a road movie, the film sees two teenage friends convince a Spanish woman to spend a few days travelling with them.
On the road, the woman teaches the teenagers about sex and growing up.
"I wanted to deal with the subject of adolescence without any sham. In Hollywood they continue to recycle the same hypocritical formula," Cuaron added.
Refund
Equally as hard-hitting was Address Unknown, which focused on the violence of a society still scarred by the Korean War.
"In my view, Korean society in the year 2001 is simmering with rage and cruelty to the point that it is about to explode,'' explained director Kim Ki-duk.
Still, if festival audiences feel the need to vent their disquiet at what they see on screen, Italian TV star Gianni Ippoliti has the answer.
Ippoliti has put up his "Give Us Our Money Back" board in the town where film-goers can write their scathing comments.
The writer of the best vitriol about a movie will be awarded the Golden Refund on 7 September.
"Whoever writes a truly negative criticism of a film that is credible and not the result of prejudices or a dislike of actors or directors will win," said Ippoliti.
Among the movies hoping not to incite a rush towards Ippoliti's board on Friday will be Monsoon Wedding.
The ironic drama is directed by Indian film-maker Mira Nair and is expected to win critical acclaim.
Also of interest will be Tosca from respected French director Benoît Jacquot who has adapted Pucccini's celebrated opera for the screen.