Director Kim Ki-duk pulls no punches in his tale of ruthless revenge and redemption, and lead actress Cho Min-soo, whom he describes as his "raven-haired Mary", is a favorite to take the best actress prize when the festival ends on Saturday.

While audiences in the West may struggle to find a theatre willing to screen Pieta, an award at the world's oldest film festival may ensure at least limited distribution, and the film is also a serious contender for the best picture Golden Lion.

The film opens with Kang-do, a lanky, expressionless debt collector terrorizing the dingy metal workshops where impoverished laborers eke out a meagre existence in a sprawling shanty town in Seoul.

Setting the plot amid squalid backstreets in the shadow of gleaming downtown skyrises, Kim makes Pieta a broader commentary on greed, the failings of the financial system and how society as a whole must take some blame for looking the other way.

"I feel that this movie in particular is a movie dedicated to humanity, to our difficult situation in an extreme capitalist crisis," Kim told reporters after a press screening of Pieta and ahead of its red carpet world premiere on Tuesday.

"That's why you may feel there have been changes in this compared with my previous movies," he added of his 18th feature film, speaking through a translator.

SHOCKING SCENES

Those debtors who cannot meet exorbitant interest costs pay dearly, with hands and arms mangled in their own machinery or limbs smashed to pieces so Kang-do can collect the insurance.

Some of those crippled take to the streets to beg, others take their own lives. One is so desperate for cash to provide for his unborn child that he offers Kang-do both hands, but before doing so has one last strum on his beloved guitar.

The cruelty ends only when Mi-sun (Cho) suddenly appears, claiming to be Kang-do's mother who abandoned him at birth.

Initially unable to accept her loving gaze and kind gestures, a suspicious Kang-do, played by Lee Jung-jin, puts her to the test with acts of extreme cruelty.

Still she comes back for more, gradually earning his affection and convincing him that her story is true.

Kim, a favorite on the European film festival circuit, took inspiration from the Western artistic tradition of portraying Mary cradling the dead Christ, most famously captured in Michelangelo's sculpture.

"I have visited the Vatican twice and seen the masterpiece by Michelangelo," he said.

"I am referring to the embrace by the Virgin Mary of her son, and the image which I've kept in my mind for many years was the idea of an embrace of the whole of humanity, the suffering and pain that are a part of human life."

Reviews of Pieta have been generally positive, while acknowledging that viewers could be put off by its graphic content.

Not everyone was surprised, however. When Kim's "The Isle" was presented in Venice in 2000, two people fainted during a screening because of a particularly gruesome scene.

"Kim Ki-duk is back in fighting form in Pieta, an intense and, for the first hour, sickeningly violent film that unexpectedly segues into a moving psychological study," said critic Deborah Young in The Hollywood Reporter.

(Additional reporting by Silvia Aloisi; writing by Mike Collett-White, editing by Paul Casciato)

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