Silvio Berlusconi has ridiculed the case against him on vice charges, saying that the number of women with whom he was alleged to have had relations defied belief.

"I am 75 years old", he told the centre-left newspaper, La Repubblica. "And even if I am a rascal, 33 girls in two months seems to me to be too many, even for a 30-year-old. It's too many for anyone."

With three weeks to go before his trial, Berlusconi was signalling the launch of a vigorous counter-offensive. In fact, Italy's prime minister is 74 years old, and the prosecutors do not claim he had sex with all the female guests who attended dinners at his mansion home outside Milan, some of whom stayed on for alleged, so-called "bunga bunga" parties.

Berlusconi portrayed them as "Carefree, elegant dinners". Asked why, in that case, the women had been paid large sums of money by his accountant, he said it was because of his charitable nature. "I pay for operations, the dentist [and] university fees to all those who have need of it", he said.

The prime minister, who split from his second wife in 2009, said that another reason for disbelieving the prosecution case was that his girlfriend would have "scratched my eyes out" if he had done the things of which he is accused. He had earlier been quoted as saying he had a steady female companion.

But, despite fevered speculation in the Italian media, she has never been identified. Berlusconi said that "fortunately, I have managed to keep her out of this dirt".

On 6 April, he is due to go on trial charged with paying an underage prostitute and then trying to cover up the alleged offence by abusing his position. The young woman at the centre of the case is a Moroccan immigrant, Karima el-Mahroug.

Documents published on Tuesday showed prosecutors believe she was only 16 years old – and not 17 as previously believed – when she first visited Berlusconi. In May 2010, she was taken to a police station on suspicion of theft, but then released after the prime minister telephoned to say, erroneously, that she was the grand-daughter of the then Egyptian president, Hosni Mubarak.

Berlusconi has insisted he believed this to be the case, and was trying to avert a diplomatic incident. But in his latest interview, and for the first time, he appeared to place responsibility for the misunderstanding on Mubarak.

"I can swear that a week before I had spoken to Mubarak about this girl for at least 15 minutes," he said.

Berlusconi added that, at the time, he was "dealing with the crisis between Libya and Switzerland. I thought: what would happen if the relative of a foreign prime minister [sic], in this case Mubarak, were to be jailed in Italy?"

In 2008, relations between Libya and Switzerland were pitched into crisis following the arrest of Colonel Gaddafi's son, Hannibal, and his wife for allegedly beating up their servants.