The leader of Italy's anti-establishment 5-Star Movement, Beppe Grillo, 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.