ROME (Reuters) - Prime Minister Silvio Berlusconi said on Monday he had no intention of stepping down and dismissed a weekend demonstration by women across Italy over his involvement in a sex scandal. Hundreds of thousands of women took part in rallies on Sunday to defend their dignity and protest over the underage prostitution scandal that has rocked the 74 year-old prime minister's centre-right government. Berlusconi has dismissed the investigation against him as "disgusting and disgraceful" but he has come under pressure from groups including the Vatican and Italy's main business lobby, Confindustria, and polls show his image has suffered. The billionaire media entrepreneur, who was badly weakened by a split in the ruling PDL party last year, told his Canale 5 network the protests were the work of his political opponents. "I saw the usual factional forces mobilised against me by a certain section of the left which uses any pretext to beat an adversary whom they can't manage to beat at the polls," he said. A survey by the Demos polling institute published in the left-leaning La Repubblica daily showed a sharp fall in his popularity ratings, which dropped 4.6 points to 30.4 percent. By contrast, Economy Minister Giulio Tremonti, widely seen as a possible alternative leader of a centre-right government if Berlusconi stepped down, saw a 7.8 percentage point rise to 50.4 percent. Milan prosecutors have requested that Berlusconi face trial over accusations that he paid for having illegal relations with a girl below the age of 18 - an offence in Italy - and that he improperly pressured police to release her from custody over theft allegations. Berlusconi said he had always treated women "with great care and respect," and added: "I have always tried to act in such a way that every woman feels special."