Italian leader Silvio Berlusconi, promising to mobilise 30,000 soldiers to combat crime after a spate of rapes in Rome, said on Sunday rapes will still occur unless there are "as many soldiers as pretty girls".

The centre-right prime minister was reelected last year on a promise to fight crime and has enlisted the help of 3,000 troops to help police. He said on Saturday he would raise that number tenfold, prompting criticism he was creating "a police state".

The opposition said three particularly brutal rape cases in Rome this month showed that mobilising the military had done nothing to reduce crime. Berlusconi responded on Sunday that "such things can always happen even in the most militarised, police state".

"We could not field a big enough force to avoid this risk. We'd have to have as many soldiers as there are pretty girls. I don't think we would manage," he told reporters.

Critics said the 72-year-old billionaire, whose fondness for flirting with women in public has earned him public reprimands from his wife, was being flippant about a very serious matter.

"Berlusconi ought to refrain from saying offensive things about women," said shadow equality minister Vittoria Franco.

"Basically what he is saying is that if women leave home alone they run the risk of being raped or attacked because it is not possible to make the country safe," she said.

Right-wing politician Alessandra Mussolini, a campaigner against court rulings that are unfair to rape victims, said: "The fact is we need so many soldiers because there are so many ugly men."