The head of Italy's Democratic Party, Pier Luigi Bersani, was in the lead after the primary to choose the main centre-left candidate in next year's national election, ahead of his main rival, Matteo Renzi, according to an exit poll on Sunday.
The exit survey by the Piepoli polling institute for RAI News 24, showed Bersani with 44 percent support, Renzi with 36 percent, and Nichi Vendola, governor of the southern region of Puglia on 16 percent.
Bersani, 61, had been favourite to win the race before voting began on Sunday, in part because of his control of the Democratic Party (PD) machine and his support among the party's core voters.
However a stronger-than-expected turnout in the primary, which was not restricted to PD members, increased expectations that Renzi, the 37 year-old mayor of Florence, would do well because of his appeal to voters outside the party.
Results are expected later on Sunday evening. If none of the five candidates in the primary wins an absolute majority after Sunday's vote, a run-off vote between the two leaders will be held on December 2.
(Reporting by Catherine Hornby and James Mackenzie; Editing by Jon Hemming)