Centre-left voters in Italy have chosen Romano Prodi as the candidate who will challenge Prime Minister Silvio Berlusconi in next year's election.
Prodi said the high turnout exceeded his rosiest expectations
The former European Commission president and Italian PM won 73.5% of votes, according to partial results.
"This is an incredible response, it's beyond every dream," he said, welcoming the high turnout.
About two-thirds of the votes have been counted so far. Final results should be announced later on Monday.
Meanwhile, in the southern region of Calabria, a regional opposition leader was shot dead at a polling station.
The police said they suspected the local mafia - known as Ndrangheta - was behind the assassination of the politician, Francesco Fortugno, but they were not ruling out any possibilities.
Almost 4 million Italians turned out to vote on Sunday, according to Italian reports.
They were asked to declare they were supporters of the centre-left and make a symbolic contribution of one euro towards the costs of the poll.
A strong mandate will increase Mr Prodi's chances of beating Mr Berlusconi in the general elections scheduled to take place in May next year.
Of the six other contestants, Fausto Bertinotti, the leader of Communist Refoundation, came second with 15.4%.
Commenting on Sunday's opposition primaries, Mr Berlusconi said: "This is the only way Prodi can win the elections: by letting only left-wingers take part in the vote."
The vote comes in the wake of a controversial electoral reform approved by parliament's lower house on Thursday.
The bill, which still needs to get the green light from the senate, reintroduces proportional representation eight months ahead of a general election.
Left-wing lawmakers have accused Mr Berlusconi of altering the law to suit his re-election prospects, at a time when his opinion poll ratings have plummeted.