* Bersani under mounting pressure to form government
* Part may split over lack of progress in stalemate
The February election left parliament split between three hostile blocs, none of which can govern alone, making an early return to the polls a growing possibility.
Speaking at a small theatre in an impoverished suburb of Rome, Bersani said the next president should be a figure of "national unity".
But he rejected forming a government with Berlusconi, saying: "A grand coalition is not the right response to the problems of the country."
Bersani has been under mounting pressure from inside and outside his party to back a government with centre-right leader Berlusconi, who is unpalatable for many PD voters, to end the political stalemate now in its second month.
The PD rally was called as a "protest against poverty" and Bersani repeated his recipe to try to create jobs and loosen the suffocating austerity that has worsened Italy's recession, which already equals the longest in postwar history.
But the political deadlock has put the PD at risk of splitting, and the need for a government has even led to an unprecedented "pact" between employers' lobbies with labour unions calling for economic reforms.
Parliament begins voting to elect a new president on Thursday. The election will be vital because constitutional rules prevent current President Giorgio Napolitano from dissolving parliament at the end of his mandate.
So Napolitano's successor, who will receive a seven-year mandate, will have another chance to find a solution that allows the formation of a government, or else he will have call new elections.
Under intense pressure, the 61-year-old Bersani was more feisty and animated during his speech on Saturday than he was during the lacklustre campaign earlier this year in which Berlusconi closed a 10-percentage-point gap and came up just short of the PD.
"After the president of the Republic (is elected), a government is needed," Bersani said.
Since failing to woo the 5-Star Movement to join him, Bersani has hoped form a minority government tolerated by enough members of the rival parties to pass a limited set of reforms. But it too would probably need the tacit support of Berlusconi.
The 76-year-old media magnate will speak later on Saturday in an outdoor rally that is expected to attract thousands in the southern city of Bari.
Comic Beppe Grillo's 5-Star Movement held a first round of online voting to select 10 candidates that it will back for president. Another online vote will be held on Monday to narrow the field down to one.
Should Bersani and Berlusconi fail to agree on a common candidate, the PD could join forces with the 5-Star or others and elect a president without the support of Berlusconi.
The prolonged stalemate has not caused the kind of market panic feared before the election, but business and labour leaders now insist on the need for a government.
Confindustria, Italy's biggest employers' lobby, and the country's top union leaders said on Saturday at a conference in Turin that they would together demand a series of reforms and the immediate formation of a government.
"We must make an accord and be united in calling for one thing: governability," Raffaele Bonanni, head of the CISL union, said.