Holding aloft banners reading "The family will save the world" and "Let'sdefend our children", a sea of people crammed into the San Giovanni square near the Italian capital's historic centre to support family values.
The square, which can hold an estimated 300,000 people, was overflowing with the young, elderly and parents with toddlers, an AFP photographer said, with many more demonstrators spilling into nearby streets. Organisers for their part said one million people were taking part. Italian police never provide figures for demos.
"In my children's schools they are talking about families made up of two fathers or two mothers, without asking parents' permission," said 41-year old doctor Giuseppe Ripa, adding: "It's dangerous and wrong."
Fellow protester Piero Uroda, a 78-year-old pharmacist, said it was "not honest to say these things to the very young, it's not like they are students who can debate these ideas."
"I don't want gay marriage or gay adoption, the natural family is like ours," he said, pointing to his relatives gathered around.
'Defending children's future'
The protest comes as the Italian senate examines a civil union bill, which Renzi wants to see go to a vote in the coming weeks, with the aim of legislation being enacted before the end of July.
The call for Italy to keep pace with its western European neighbours on the issue has grown stronger since Ireland voted overwhelmingly in favour of gay marriage last month.
But rights activists warn it will be no mean feat for the leader of the centre-left Democratic Party to find allies across the political spectrum, blaming an excessive influence of the Catholic clergy on the political class.
While a survey published by Italian daily La Stampa at the end of May found that 51 percent of voters would support gay marriage, protesters in Rome said the state should stop trying to interfere in sexual or gender education.
"We are asking for families based on marriage be respected, and stressing the central role parents play. We forcefully reject the attempt to sneak into the curriculum projects which aim to destroy children's sexual identities," said Massimo Gandolfini, spokesman for the "Defend our children!" committee.
There were both cries of support and derision from the political sphere. Matteo Salvini, rising star on the far right and head of the Northern League party, sent a Facebook "hug for all the mummies and daddies protesting peacefully in Rome to defend their children's future."
Vincenzo Brana, head of the long-standing Arcigay gay rights association in Bologna, was quick to point out that "today is World Refugee Day", with hundreds of thousands of migrants in Italy facing an uncertain future.
Their problems "are an urgent issue which concerns us all. Those who take to the streets to talk of other things show they are living on another planet, and I would recommend leaving them on that planet alone."