Former European Commission President Romano Prodi looks set to be crowned as opposition candidate to run against Italian Prime Minister Silvio Berlusconi in general elections next spring.
Early exit polls in the vote by centre-left voters gave Mr Prodi 75 per cent of the vote, a landslide victory over the six other candidates drawn from across the centre-left spectrum and far higher than anticipated.
However, Sunday's poll was overshadowed by the mafia-style killing of Francesco Fortugno (54), the vice president of the regional government of Calabria in the south of the country, as he exited a polling station.
Politicians from across the political spectrum denounced the murder - the first of its kind in Calabria in 16 years - and Interior Minister Giuseppe Pisanu said he would fly to the region early today.
Turnout at the 10,000 polling booths far exceeded expectations, with organisers predicting more than three million voters by the time stations closed at 10 p.m., three times more than forecast.
Centre-left leaders said the success of the primaries had weakened Mr Berlusconi and showed Mr Prodi had won the country's backing to oust the billionaire media tycoon after his five years in power, Italy's longest government since World War II.
Mr Berlusconi replied Mr Prodi should enjoy his victory because it was the only one he was going to win.