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ROME — Hundreds of thousands of Italians descended on the capital city Saturday to protest a bill that would legalize same-sex unions across the country.

Lawmakers will vote next week on the proposals, which would give gay couples with civil unions — and unmarried heterosexual couples — the same rights as they would have under marriage, including the adoption of their partners' children.

“Italy can’t remain the tail-end in Europe in the field of civil and human rights,” Monica Cirinna, the bill’s author, told NBC News. “We can’t go to Europe to complain about economic problems if we carry such a heavy burden as the failure to recognize same-sex couples.”

A child holds a sign reading "It is wrong even if it becomes law" during a rally against same-sex unions and gay adoption in Rome on Jan. 30. REMO CASILLI / Reuters

Italy is the only Western European nation that doesn’t recognize same-sex civil unions or gay marriage, in part because of the heavy influence of the Catholic Church on government and public opinion.

Pope Francis last week warned that “there can be no confusion between the family willed by God and any other type of union.”

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Many of those opposed to the change fear the law is a Trojan horse that would lead to full legalization of gay marriage. Some fear it could also open the way to loosening laws on surrogate motherhood, which is illegal in Italy.

Saturday's protest was billed by supporters as a "Family Day" in the name of traditional marriage. It has been backed by Cardinal Angelo Bagnasco, president of the Italian Bishops’ Conference, The Tablet reported.

Upwards of an estimated half-million people filled Rome's Circus Maximus venue, although organizers said as many as 2 million supporters attended the rally. That number could not immediately be verified by NBC News.

Rally-goers opposing the same-sex unions bill were adamant about their positions.

"We are against this law because children are not a given right," said Stefano Mariani. "Since the beginning of time, children have come from a mother, and a father."

"Two men can’t give birth to life, so you can’t call that a family. It’s selfish of them to want children just to satisfy their desires," added Pamela Burattini.

But last weekend, hundreds of thousands of supporters of the bill flooded more than 100 cities with rainbow flags.

A smaller group of supporters gathered in front of Senate in Rome on Thursday while the bill was being debated. Many held ringing alarm clocks, chanting: “Wake up Italy.”

Supporters of the bill legalizing same-sex unions gathered outside Italy's Senate on Thursday. Claudio Lavanga / NBC News

“As an Italian, the right to officialize our relationship is the same as the right to exist,” said Andrea Masini, who was at the rally with his partner, Christophe Betmalle. “Our friends and families know us as a couple. Our nephews call us uncles. But for the state, we can only be two friends. Why shouldn’t we be officially recognized?”

Civil unions have been on the Italian government’s agenda in some form or another since 1988, but this is the first time a law has a real chance of being approved.

“Last year, the European Court of Human Rights has condemned Italy for failing to protect the rights of same sex couples,” said Gabriele Piazzoni, the leader of Italy’s LGBT association, ARCIGAY. “If a gay couple gets married abroad, comes to Italy, and their rights are not recognized, they can get sue the government for 5,000 euros. And the state has to give it to them, because Europe recognized it’s a violation of their rights. This is one of the reasons the government is proposing this law, because they are being pressured to do it by the European Union.”

The bill is widely expected to be approved in the Senate by Feb. 9, despite being opposed by large numbers of center-right opposition lawmakers and divisions within the ruling Democratic Party.