The leader of Italy's anti-establishment 5-Star Movement, Beppe Grillo, on Sunday criticised the re-election of President Giorgio Napolitano as a desperate attempt to retain power by a discredited establishment.
The euro zone's third largest economy is still without a government two months after a general election, has scarcely grown in 20 years and is grappling with the highest level of unemployment in decades.
Talks on the formation of a new administration are expected to resume in the coming week, with the parties under pressure from Napolitano to reach a deal.
The broad, right-left agreement to hand Napolitano another seven-year mandate could end Italy's political impasse, which resulted after no single force emerged from February's election with a workable majority in parliament.
Grillo's movement backed Stefano Rodota, a leftist academic, while Napolitano was elected by Silvio Berlusconi's centre-right bloc, outgoing Prime Minister Mario Monti's centrist movement, and the badly divided centre-left Democratic Party (PD).
Grillo, whose vow to kick out the old guard allowed his 5-Star Movement to win one in four votes in its first national election, called the presidential vote "a cunning little institutional coup" designed to keep the old parties in power.
"They have stolen a year of time. I don't think we can accept this," he told a press conference, his first since the February election. Grillo will lead a protest rally against the result in Rome later on Sunday.
The 87-year-old Napolitano's re-election - the first time a president has been asked to serve a second term - gives him the option of calling a snap parliamentary election, allowing him more leverage to pressure the parties to reach a deal.
Napolitano now has the power to dissolve parliament, which he did not have in the final months of his first term.
He is likely to spell out his strategy in an address to parliament on Monday, but he has already made it clear that he favours forming a government to avoid a potentially destabilising new election.
Rocco Buttiglione, a lower house lawmaker and high-ranking member of Monti's Civic Choice bloc, said he expected an agreement on a government within a week.
"Now we must work to construct a broad government that will essentially be a replay of the Monti government," Buttiglione told Reuters.
"This government must last years, not months. If we go back to elections before the bitter medicine has had time to take effect, it's clear that voters will back Grillo," Buttiglione said.
Thousands protested angrily outside parliament after Saturday's presidential vote by lawmakers, some shouting the name of Rodota.
Grillo backed out of a plan to join the protests himself, saying on Sunday that he had feared the situation could turn violent, and toned down the terms in which he condemned the vote, which he had initially denounced as a full-blown "coup d'etat".
On Sunday, commentators quickly began speculating on what the next government could look like should Napolitano succeed in pressuring the parties into forming a government.
The Democratic Party's leadership resigned after party rebels sabotaged attempts to get their candidate elected president, and the party will now have to chose a new leader to take over from Pier Luigi Bersani.
Newspaper reports named deputy leader Letta as one possibility to lead the party and help to form a government, but this was disputed by the newly resigned president of the party Rosy Bindi.
"I have great esteem for Enrico Letta and I believe that he would be very capable and would know how to run a government, but this is certainly not the moment," Bindi told SkyTG24 television.
A priority of any government is likely to be the reform of the electoral law which awards a large bonus to the party that wins the most votes in the lower house, but not in the Senate, which was partly responsible for the deadlock. Any election held under the same law could return a similar inconclusive result.
The idea of a broad coalition government had so far been rejected by the centre-left, but disarray in the party's leadership means they may be eager to avoid a quick return to elections and more amenable to a deal.
The result of Saturday's vote was widely heralded as a victory for Berlusconi. The 76 year-old media magnate, who was forced from office at the height of the debt crisis in 2011 and was still being written off until shortly before February's election, now leads in opinion polls.