Beppe Grillo, the former comedian whose anti-establishment Five Star Movement (M5S) holds the balance of power in Italy's dysfunctional new parliament, has angrily condemned a group of MPs he claims broke party lines in one of their first major votes.

In a sign of growing tension between the movement's co-founder and some of its 163 new representatives, Grillo demanded that choices at the ballot box be made public after a centre-left candidate was elected speaker of the Senate with the help of a handful of M5S votes.

He said that a majority of the M5S's parliamentarians – or grillini, as they are known – had agreed in advance that they would submit blank votes in the runoff between Pietro Grasso, a former anti-mafia prosecutor standing on behalf of the Democratic party (PD), and Renato Schifani, the candidate put forward by Silvio Berlusconi's Freedom People party (PdL). Any MP who had not done so, wrote Grillo on his blog, would be expected "to take the necessary consequences" for having "lied to voters".

Grasso, who put hundreds of people behind bars in his years spent battling the mafia, was elected in a secret ballot on Saturday evening. Vito Crimi, the M5S's group head in the Senate, said that, while the majority of grillini senators had indeed entered blank votes, some had voted "according to their conscience" – a measured stance that seemed at odds with Grillo's. The M5S's overriding concern, added Crimi, had been "not to vote for Schifani".

Underneath Grillo's blog, many M5S supporters expressed their disappointment at his attitude towards the Grasso vote. "Dear Beppe, you created a wonderful thing," wrote one commenter who gave his name as Enrico Sodini. "But I don't understand now what the aim is … euthanasia?"

The election of Grasso and his counterpart in the chamber, Laura Boldrini, a former spokesman of the UN high commissioner for refugees who is now representing the leftwing Left Ecology Freedom (SEL) party, marked the first acts of the fractured and chaotic new parliament, which convened for the first time on Friday. The prospects of a stable government emerging from the aftermath of last month's inconclusive election, however, remain slim.

On Sunday, the man who will lead the negotiations to try to make that happen, the president, Giorgio Napolitano, appealed to political leaders to overcome their divisions for the good of the country. In remarks timed nominally to mark the anniversary of Italian unification but with clear relevance to the current impasse, the 87-year-old head of state called on them to show a "sense of responsibility" and to avoid splitting into "factions".

Napolitano will on Wednesday begin the unenviable task of beginning talks with party leaders to see if a government can be formed. The gridlock is daunting. Pier Luigi Bersani, whose PD has an outright majority in the chamber but not in the senate, has been making overtures to the M5S which have been flatly rejected by Grillo – who, although not in parliament himself, remains the movement's key figure. Berlusconi has indicated his openness to a grand coalition with the centre-left – a prospect that Bersani appears to have rejected out of hand.