"The only point of this electoral reform proposal is to block us because we are the danger to the system," Grillo, whose party won a quarter of the vote at last year's national election, told a gathering of the foreign press in Rome.
The centre-left's dynamic new leader Matteo Renzi this week drew up a plan to change the voting rules blamed for Italy's chronic political instability after reaching a widely contested deal with centre-right leader and former prime minister Silvio Berlusconi.
The 5-Star Movement, which espouses an eclectic mix of green and anti-establishment policies and wants a referendum on Italy's euro membership, has stayed in opposition since the February 2013 vote and refuses to collaborate with the left-right coalition government.
Grillo dismissed Renzi, who is likely to lead the centre-left Democratic Party into the next election as its candidate for prime minister, as a "cartoon character".
He said the pact with Berlusconi, who is barred from parliament after a tax fraud conviction, was a stitch-up between parties that only pretend to be adversaries.
"What kind of country is this? You can't reform the electoral law in some night-time deal between a criminal and a cartoon character, you have to do it in parliament," Grillo said in his usual caustic tones.
"Renzi and Berlusconi are the same thing, they are interchangeable, they represent the same interests," he said.
Grillo expressed confidence that the deal which the Chamber of Deputies will start debating next week will not be approved, partly because it includes the abolition of the upper house Senate as an elected chamber.
"Can you imagine all these senators voting to abolish themselves in secret votes?" he quipped.
The electoral reform proposed by Renzi and Berlusconi offers a large winner's bonus of seats to the largest grouping and would therefore penalise Grillo's movement, which rejects alliances with either of the two main blocs.
After consulting its members online, 5-Star is pushing for an electoral system based on proportional representation, without the winner's bonuses envisaged by both the present system and reform plan.
Remain Italy's largest party?
The 5-Star Movement has fallen out of the limelight since it shot to success at last February's vote, but it retains a sizeable core of support and remains a potential key player.
It has few friends in the domestic media, which Grillo called "the real cancer of our system," and as a purely opposition force it has had no direct impact on government policy. It has also fared badly in several local elections.
However, according to opinion polls it still gets around 20% support.
"We may still be Italy's largest party," Grillo said, adding that he expected a major success at elections for the European Parliament in May where a vote for 5-Star will enable Italians to show their dissent with European austerity policies.
"I am telling you now that we will win the European elections," he shouted to the packed news conference.
Centre-left Prime Minister Enrico Letta has warned of a likely strong showing for 5-Star at the European elections unless Italy can revive its stagnant economy and persuade Europe to adopt a more growth-friendly agenda.
Despite a typically spirited performance during a nearly three-hour news conference, Grillo, 65, showed some signs of battle fatigue as he defended his policy of uncompromising non-collaboration with the mainstream parties.
"We can't do deals with these cheats and hypocrites," he said. "But if at the end of the day the Italians want to vote for swindlers, then we'll just have to live with the swindlers and I'll be happy to go and do something else."