* 5-Star Movement ramping up attacks on traditional parties
* Grillo accused of sexism, instigating violence
* Party's support remains strong despite criticism
By James Mackenzie
ROME, Feb 3 (Reuters) - Italy's anti-establishment 5-Star Movement faced strong criticism on Monday after attacks and online insults against the speaker of the lower house Laura Boldrini, who responded by branding her critics "potential rapists".
Boldrini's retort followed days of escalating tensions in parliament where deputies of Beppe Grillo's Movement, angry over moves to push through approval of government decrees, have blocked committee work and clashed with rival parties.
The confrontation has fed a venomous climate in parliament just as it has begun debating proposals to overhaul voting rules blamed for the chaos of last year's deadlocked election, which forced the two main centre-left and centre-right blocs into a fragile and unstable coalition.
The 5-Star Movement, which has ridden a wave of anger against the mainstream political parties after years of scandal and economic stagnation, has called for both the impeachment of President Giorgio Napolitano and the removal of Boldrini as speaker of the lower house, accusing her of bias.
She particularly angered the party by imposing a guillotine to speed up passage of decrees on the recapitalisation of the Bank of Italy and a housing tax after 5-Star deputies had tried to stop approval by blocking access to committee rooms.
But online comments on Grillo's popular blog have gone further, culminating in an online video showing a man driving a car with a cardboard cut-out of Boldrini in the passenger seat entitled "What would you do alone in a car with Boldrini?"
The post attracted numerous offensive comments, many violent in tone, before they were removed by 5-Star officials.
"It is instigation to violence, you just have to look at the comments. It means that people who contribute to this blog don't want to have a discussion on matters of substance but to offend and humiliate, they are potential rapists," Boldrini told RAI state television on Sunday.
She said the party's tactics in parliament, where disputes have come close to physical fights, were "subversive".
Grillo called Boldrini a "hypocrite" for not condemning a deputy from Mario Monti's Civic Choice who was caught on film striking a 5-Star female deputy in the face during a chaotic episode in the chamber last week when 5-Star politicians tried to clamber onto the benches reserved for the government.
Senior politicians, including the president, Prime Minister Enrico Letta and the new head of the centre-left Democratic Party (PD) Matteo Renzi, have condemned the 5-Star tactics.
"These acts are like those used by fascist squads," Renzi said in an interview with La Repubblica newspaper, adding that the online insults against Boldrini were "squalid".
Reaction from the 5-Star Movement, which remains one of the three biggest parties in Italy alongside the PD and former prime minister Silvio Berlusconi's Forza Italia party, has been a mixture of embarrassment and fury.
"Vulgarity and insults should always be condemned with no 'ifs' and 'buts'," said 5-Star Senator Serenella Fucksia.
"Some people use false names. They might not even be Grillo supporters but Renzi supporters," she told La Repubblica, adding that Boldrini was biased, inadequate and should resign.
While 5-Star's tactics have also been condemned by Italy's mainstream press, polls suggest its support remains around 20 percent or higher. And unscientific viewer surveys by both Sky Italia and state broadcaster RAI showed large majorities of participants supported its increasingly aggressive opposition. (additional reporting by Gavin Jones; Editing by Gareth Jones)