The disturbing anti-mafia bestseller "Gomorra" that provoked death threats against its Italian author makes its debut in film form in Cannes on Sunday.

Roberto Saviano's expose of the Naples-area Camorra mafia has sold more than a million copies since the book was published in Italy in 2006.

From gangland warfare to the Camorra's infiltration into the building industry, arms and drug trafficking, and even the mishandling of toxic waste for a profit, Saviano paints a terrifying portrait of a crime syndicate that will stop at nothing.

The revelations of Saviano, born in a poor section of Naples 28 years ago, have enraged some mafia bosses.

Police discovered a plan to eliminate the young author and placed him under protection at the end of 2006.

"I am constantly escorted by police, I move house all the time," Saviano told the daily La Repubblica this month. "I don't have a normal life anymore."

He added: "What the mafia couldn't forgive was not the book but its success. If the book hadn't been published in Naples, I think it would have been okay, the mafiosi would even share it among themselves because they would be pleased that someone told their story."

Thirty-three foreign publishers have bought translation rights to the book, according to publisher Mandadori.

"Gomorra" has also been made into a play.

Less than a decade older than Saviano, 39-year-old filmmaker Matteo Garrone decided soon after the book came out that he wanted to turn it into a movie, choosing to focus on five characters whose paths crossed with the Camorra.

"It's an apocalyptic, hopeless film," said Garrone, who shot it in utmost secrecy in Naples mainly with non-professional actors in poor neighbourhoods including the Camorra bastion of Scampia.

"Don't think of it as a classic expose film pitting good against evil ... because in reality things are more complicated and the boundaries are less clear," Garrone told the leading Italian daily Corriere della Sera.

"I reveal the biggest open-air drug market and the dynamics of gangland warfare after a truce that lasted years," he said.

"I shot very violent, atrocious scenes, but the one that moved me the most shows two youths who have always been friends but are forced to go separate ways because they belong to different clans."

The common denominator of the film's characters is "a humanity that is conditioned by a system, a machine that crushes you and that you cannot rebel against," Garrone said.

It will be Garrone's second entry at Cannes, after he showed "L'Imbalsamatore" (The Embalmer) in 2002 in the Directors' Fortnight category.