Thousands of protesters have streamed into a piazza in central Rome to show their support for government proposals to grant rights to gay couples.
The Vatican sees the bill as an attack on the traditional family
Waving rainbow coloured-flags, the 50,000-strong crowd voiced support for a bill giving gay couples legal status.
"Wake up, it's time for our rights!" some shouted. Others donned bishop's mitres with anti-Vatican slogans.
The bill was approved by Italy's cabinet last week but faces a tough battle through parliament.
Prime Minister Romano Prodi, who promised legal rights for de facto couples in his 2006 election campaign, approved the proposals on 8 February.
The highly controversial move came after months of heated debate in the broad, ruling coalition and fierce opposition from the Vatican.
But correspondents say that opposition from an already fractured coalition, may mean the proposal does not become law.
Leftist coalition members including the Communist and Green parties have been campaigning in favour of the proposals but they are opposed by some Christian Democrats.
Pope Benedict XVI has also spoken out against what the Vatican sees as an attack on the traditional family.
"It's a demonstration to unite and not to divide, and it's the biggest rally ever held in Italy to demand a law on civil unions," activist Alessandro Zan told the crowd.
Two ministers spoke at the rally and centre-left deputy Franco Grillini, also a gay rights campaigner, said that the proposed law paled beside those of other European countries.
"None of the countries has seen the apocalyptic forecasts about the fate of the traditional family come true," Mr Grillini said.
If parliament passes the package, unmarried couples will get greater health and social welfare benefits.
But partners will enjoy inheritance rights only if they have been living together for at least nine years.