Silvio Berlusconi's trial on charges of sex with an alleged underage prostitute and abuse of power opened on Wednesday as the storm over the 74-year-old Italian premier's private life finally landed in court. Berlusconi himself did not attend the hearing and the judges immediately adjourned the trial until May 31. The woman at the centre of the case, Karima El Mahroug -- nicknamed "Ruby the Heart Stealer" -- was not present either. The hearing at Milan's main court building lasted just under 10 minutes and a handful of protesters rallied outside both for and against Berlusconi. The prime minister said in a letter given to the court by his defence lawyers that he would have wanted to take part in the hearing but that he had "institutional engagements" in Rome that prevented him from doing so. The defence says it will call US actor George Clooney and Real Madrid football star Cristiano Ronaldo to testify -- in a bid to undermine comments about both men made by the alleged prostitute during questioning. Foreign Minister Franco Frattini is also reportedly on the witness list. The 33 women named by the prosecution as having taken part in masked orgies and erotic dances with Berlusconi are also expected to testify but a lawyer for Mahroug said "Ruby" would not be a claimant or injured party in the case. "That would contradict what she has always said -- that she did not have sexual relations with the prime minister," said Paola Boccardi, her lawyer. Feminist group Arcidonna said it had made a request as a claimant in the case against Berlusconi for having "damaged the dignity of women". The billionaire tycoon is no stranger to the dock after years of investigations and trials linked to his business interests but this will be the first time that his tumultuous personal life is exposed in a courtroom. "I don't think any Western democracy has ever seen a case like this," Antonio Padellaro, editor-in-chief of Il Fatto Quotidiano, the newspaper that first broke the story about the scandal last year, told AFP in an interview. "The latest polls show a major plunge in Berlusconi's popularity, who is clearly suffering the consequences of his behaviour and his lifestyle," said Padellaro, as the prime minister's ratings fall to record lows. The trial has attracted intense domestic and international media interest, with more than 100 journalists from around the world attending the hearing. The twice-divorced Berlusconi has made no secret of his love for beautiful young women but vehemently denies having sex on several occasions last year with then 17-year-old nightclub dancer Mahroug. Berlusconi also says he ensured she was released from police custody after she was detained for theft in order to avoid a diplomatic incident because he thought she was the niece of former Egyptian president Hosni Mubarak. Wednesday's hearing comes after months of wire taps by prosecutors in Milan in a mafia-style investigation that fills 20,000 pages of files and details of which have been splashed across Italian tabloids. The underage sex charge carries a maximum sentence of three years, while the abuse of power accusation is punishable by up to 12 years in prison. Berlusconi has laughed off the charges, which he describes as "absurd". "I'm nearly 75 years old and even if I am naughty... 33 women in two months seems a bit too much for me," the prime minister joked in March. But his ratings have been hit, plunging to an all-time low of 33 percent in March according to a poll by Italy's IPR Marketing Institute. Hundreds of thousands of women took to the streets in Italy to protest against the playboy tycoon after the scandal first broke, and rejoiced when it transpired the ageing Latin lover would be judged by a panel of three women. The "Ruby" trial is the third ongoing trial involving the prime minister -- another is for tax fraud linked to his Mediaset business empire and a second is for alleged bribery of British lawyer David Mills to give false testimony. Berlusconi has denied all charges and has accused prosecutors of conspiring against him in a left-wing plot. In typically flamboyant style, he said last month: "I am the most accused person in history and in the universe."