The editor of the newspaper that broke the story about an inquiry linked to Prime Minister Silvio Berlusconi having sex with an underage prostitute said the trial starting Wednesday will be "exceptional".

"I don't think any Western democracy has ever seen a case like this of a prime minister in office faced with accusations like prostitution with a minor," Antonio Padellaro, editor-in-chief of Il Fatto Quotidiano, told AFP.

Berlusconi is accused of having sex last year with a then 17-year-old Karima El Mahroug, better known as "Ruby the Heart Stealer" and then abusing his powers by getting her released from police custody in a separate incident.

Il Fatto Quotidiano, an independent leftist daily founded in 2009 that has already made its name with numerous journalistic investigations, first published a story with initial details about the inquiry in October 2010.

"Wednesday will be an exceptional day for Italian politics and for the entire world, which is curious to know exactly how the trial will end, whether Silvio Berlusconi will be found guilty or not," Padellaro said.

Berlusconi has denied ever having sex with Moroccan-born El Mahroug.

He said that what made this case stand out from Berlusconi's many previous legal woes was the "mass of proof accumulated by the prosecutors in Milan" -- including hundreds of pages of testimony and telephone wire taps.

Prosecution documents leaked to Italian newspapers in recent days show some 43 women including "Ruby" are alleged to have taken part in raunchy parties at Berlusconi's private residence near Milan that then ended with sex sessions.

"He behaved in such an obvious and flagrant way!" Padellaro said.

"He is already defending him in his usual way, by constantly approving laws that can defend him from prosecution," he added.

Berlusconi has long accused prosecutors of being out to get him and of having a left-wing political bias and his government has proposed an overhaul of the justice system that critics say would curb judges' powers.

"His entourage talks about political and judicial persecution. Persecution? Yeah right!"

Padellaro also criticised the list of witnesses called to testify by the defence including US actor George Clooney and Real Madrid football star Cristiano Ronaldo -- aimed at undermining El Mahroug's testimony.

"We'll see if they want to testify but the idea is to transform the trial into a sort of show in which he would come out clean... like a paternal figure who is generous to young women," he said.

Irrespective of the outcome of the trial, it appears that Berlusconi's popularity ratings have already been hit. In the latest poll for March his rating had fallen to a record low of 33 percent.

"His popularity has clearly gone down," Padellaro said.

"The latest polls show a major plunge in Berlusconi's popularity, who is clearly suffering the consequences of his behaviour and his lifestyle."

The editor also criticised the focus on "amusing" aspects of the case.

"For Italians who have had to deal with him for 17 years, there's nothing funny. In 17 years he has transformed Italian politics with the enormous amount of money that he has put on the table," he said.

"He has manipulated and destroyed the political system that in a normal democracy should be a fair confrontation of ideas, of political projects, of personalities."

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