Opinion polls taken in the few days leading up to election day showed the so-called Olive Tree coalition with around 46 per cent of the vote, some five points ahead of the Freedom Alliance led by Silvio Berlusconi and his reformed neo-fascist ally Gianfranco Fini.
The forecasts also gave a strong showing to the far-left Rifondazione Comunista and to the separatist Northern League, either of whom could end up holding the balance of power in a hung parliament.
A victory for the centre-left would be a momentous event in a country where the main left-wing group has been excluded from national government since the end of World War Two.
Supporters of the Olive Tree watching television coverage on a giant video screen in Rome's Piazza Santi Apostoli let out a loud roar when the forecasts were flashed up. "It seems that we have won," said one left-wing deputy, Luigi Berlinguer.
Political leaders, however, treated the polls with extreme caution, not least because Italian voters are notorious for lying about their intentions.
Full results were expected during the course of the night.
Campaign trail, page 8