It has been 2,000 years since Romans gathered at the Circus Maximus to watch chariots roar around the racetrack, but a new battle was brought to the ancient site on Saturday .

Clutching banners reading “We defend our children” and “The family is founded on marriage between a man and a woman”, hundreds of thousands of people gathered for a “Family Day” to protest against a proposed bill to legalise same-sex unions.

“We have a lot of grandchildren; their future is ruined if marriage is destroyed,” said protester Ida. “Children have a right to have a mother and a father. To grow up healthy, they need a male and a female figure.”

The law, currently facing fierce debate in the senate, is aimed at granting legal rights to same-sex couples for the first time in Italy. Among the provisions are civil partnerships, the right to inherit a partner’s pension and a controversial “stepchild adoption” article which would allow a child legally to have same-sex parents in the case of a gay person adopting their partner’s biological child.

There were both patriotic and religious overtones to the Rome rally. Italian flags were sold and religious banners waved while priests and nuns walked among the crowds. “I believe in Christian and family values. Our children are not for sale,” said Gianluca, a participant.

Organisers estimated that two million people attended the protest, a figure which could not immediately be confirmed by Italian police. Young volunteers were on hand to help and numerous families laid out picnic blankets and rolled buggies on to the grass.

It is no coincidence that while same-sex couples have been granted rights in other Catholic-majority countries, such as Ireland and Spain, the home of the Vatican remains the last country in western Europe not to offer rights to gay couples.

The Irish referendum last year to legalise gay marriage was deemed a “defeat for humanity” by Cardinal Pietro Parolin, the Vatican’s secretary of state. The new bill represents an immense challenge to the authority of the Catholic church at a time when its influence over Italian politics and society is seen to be diminishing. The cause has also rallied conservatives of all stripes, who argue that the law betrays the “natural family” and the Italian constitution.

“In Italy the constitutional court said marriage is only between a man and a woman ... This law is a scam, because it requires changing the constitution,” said Gianfranco Amato, one of the organisers of the protest.

In fact the Italian constitution refers only to “spouses” rather than specifying gender, although the court ruled in 2010 that banning same-sex unions did not breach citizens’ constitutional rights. While the new bill makes no mention of the word “marriage”, another protest organiser, Toni Brandi, argued same-sex unions are “a direct assault” on the tradition of marriage.

The protest camp has been eagerly awaiting remarks from Pope Francis, who has failed to comment directly on the law but made his view on the issue clear. “There cannot be confusion between the family wanted by God and every other type of union,” he said earlier this month.

Those against the law have found greater support in Cardinal Angelo Bagnasco, head of the Italian bishops’ conference, who said having children should not be considered a right. “Children have the right to grow up with a mother and a father. The family is an anthropological fact, not an ideological one,” he added.

But the church’s influence is seen as markedly weaker than in 2007, when it successfully campaigned for politicians to reject a same-sex union bill. “Family Day” organisers have been vocal in stating the event has been organised by citizens, not the church, and said before the rally the range of protesters would include pagans and Muslims.

As senators began debating the bill on Thursday morning, a thick fog hung over Rome which saw St Peter’s Basilica vanish from the cityscape. But Alessia Crocini, who has a child with her female partner, Chiara, said the strength of the church’s opposition was still palpable in the Eternal City.

“We’re a symbol, the final stronghold. It’s as if the church will collapse [if the law passes], the Vatican will fall, St Peter’s will crumble!” she said. “So there’s very strong resistance.”

The Italian bill comes nearly six years after Argentina – where Pope Francis served as archbishop of Buenos Aires – legalised same-sex marriage. Cardinal Jorge Mario Bergoglio, as the pope was then known, was openly opposed to the law, although campaigners have claimed the pontiff secretly supported same-sex unions.

Beyond the church, in Italy the debate has also reached into the business world, with train company Italo offering a 30% discount to passengers travelling to Rome for “Family Day”. The firm argued it was a “purely commercial” decision. The move is not without precedent; in 2013 the chairman of pasta maker Barilla said he would never feature a gay family in his adverts. If customers didn’t like his approach they could buy another make of pasta, Guido Barilla said.

But there have also been signs of the tide turning. Public pressure forced Barilla to issue an apology, while last year the D&G fashion house faced a boycott after designers Domenico Dolce and Stefano Gabbana made negative comments about same-sex families. On Saturday the same designers released a new line of handbags and T-shirts celebrating gay families.

In politics the key opponent to the law has been interior minister Angelino Alfano, a former protege of Silvio Berlusconi, who recently said that stepchild adoption risked bringing the “repugnant, illicit trade” of surrogacy to Italy. Alfano sits in an uneasy coalition with Italy’s leftwing prime minister, Matteo Renzi, who has urged lawmakers to back the bill.

“Society and the people are much more ahead than the politics,” said Crocini. “In social situations we never have bad reactions, are offended or suffer discrimination.”

Local politicians have been taking the boldest steps, with mayors defying Italy’s ban on same-sex unions by including gay partnerships contracted abroad in the city hall register. The move was deemed illegal but nonetheless spurred the debate and showed a society ready for change.

“Family Day” undoubtedly demonstrated that strong opposition exists, but the rally came a week after a “Wake up Italy!” mobilisation gathered together gay rights supporters in around 100 piazzas across Italy.

And while the drive to push same-sex unions through parliament failed nearly a decade ago, this time campaigners have the backing of the European Court of Human Rights. Last year the court ruled in favour of three gay couples who claimed that Italy breached their rights in failing to offer them adequate legal recognition, forcing them to live double lives.

Crocini said she lives “a complicated life” whereby she needs a document signed by Chiara in order to legally pick up their baby, Levon, from nursery. “According to the law, Levon only has one parent – Chiara – so he’s the son of a mother who casually lives with another person,” she explained.

It has been suggested that Renzi could compromise on the bill, scrapping stepchild adoption in order to push civil partnerships through, but Crocini and LGBT groups have said this would be a complete failure.

“The main hope is that the law passes [without changes], which would really be a small step for now, the first step towards equality,” said Crocini. Those supporting the bill have vowed to fight on for full marriage rights. They will face a tough battle. Conservatives have vowed to gather 500,000 signatures to win a referendum on the current proposals if they are passed.

“Today in Italy homosexual couples already have all the rights,” claimed Amato, arguing that the bill is merely a distraction from more serious problems. “As if Italy doesn’t have problems of employment, of Isis, of public order. The country is full of problems and parliament talks about stepchild adoption – it’s absurd.”