Prodi strengthens claim to be Berlusconi's main rival

The elections are designed to endorse the the challenger to Mr Berlusconi at the general election due in spring 2006. Participation was meant to be restricted to regular supporters of the left-wing parties, but as no proof of affiliation was required, anyone could vote. Voters were asked to pay one euro each towards election costs.

Mr Prodi, an economics expert and former Christian Democrat popularly known in Italy as "the Professor", beat Mr Berlusconi to head a centre-left coalition in 1996. He fell from power two years later after a revolt by the Rifondazione Comunista party, led by Fausto Bertinotti.

In March 1999 he was appointed the president of the European Commission by the European Council, a post he left in 2004.

Mr Bertinotti, one of six rival left-wing candidates standing in yesterday's election, remains the most potent threat to Mr Prodi. Piero Fassino, the leader of the biggest opposition party, the Democrats of the Left, said: "When the polls shut Berlusconi will be looking at only one result: how many votes will Romano Prodi get, because that is who his adversary will be in 2006." Mr Fassino has thrown his party's weight behind a Prodi candidacy.

But Mr Prodi was dealt a blow by Mr Berlusconi last week when the Prime Minister introduced a reform of Italy's electoral system to parliament. It has already passed the lower house and is expected to sail through the Senate, too.

The reform replaces Italy's present hybrid system, a mix of first-past-the-post and PR, with a full-blown PR system based on the electoral weight of the parties. Mr Prodi's problem, perhaps unique among democratic aspirants to power in the Western world, is that he does not belong to a party. In the old system, that did not matter. In the new system, belonging to a party is the only thing that does matter.

Before the start of voting, Mr Prodi exhorted his supporters to come out and vote in large numbers. "After this undemocratic electoral reform, we need to show that we are strong and have faith in the future," he said.

In the afternoon, after hearing that the turnout had reached 1.3 million, he said: "Democracy has already won. Italians don't accept what has happened in the past few days."

The election was overshadowed by the murder of a senior politician in the southern region of Calabria as he was at his local polling station waiting to vote.

Francesco Fortugno, the vice-president of Calabria's Regional Council and a member of the centre-left Margherita party, was shot five times at close quarters by two masked gunmen with pistols. He died on the way to hospital.

Calabria is home to the 'Ndrangheta, a local version of the Mafia, which is said to have become wealthier and more powerful than its counterpart in Sicily.

If Mr Berlusconi's plan to build a suspension bridge from Calabria to Sicily goes ahead, it is feared that the gangs on both sides of the strait will be able to use the link to expand their network. The plan is opposed by the opposition party.

The regional council of Calabria recently sued the 'Ndrangheta for millions of euros, alleging damage to the image and economy of the region.