Greece legalized same-sex marriage and equal parental rights for same-sex couples on Thursday as lawmakers passed a bill that has divided Greek society and drawn vehement opposition from the country’s powerful Orthodox Church.

Although Greece became the 16th European Union country to allow same-sex marriage, it is the first Orthodox Christian nation to pass such a law. The country extended civil partnerships to same-sex couples in 2015, but stopped short of extending equal parental rights at the time.

Prime Minister Kyriakos Mitsotakis had pledged to pass the new measures after his landslide re-election last year. He told his cabinet last month that same-sex marriage was a matter of equal rights, noted that similar legislation was in place in more than 30 other countries, and said that there should be no “second-class citizens” or “children of a lesser God.”

In addition to recognizing same-sex marriages, the legislation clears the way for adoption and gives the same rights to both same-sex parents as a child’s legal guardian, whereas to date such rights have applied only to the biological parent. It would also affect the daily lives of same-sex couples, Mr. Mitsotakis told Parliament on Thursday, allowing those with children “to collect them from school, to be able to travel with them, to take them to the doctor.”

The law does not provide same-sex couples with access to assisted reproduction or the option of surrogate pregnancies. It also does not give transgender people rights as parents.

The bill passed with 176 votes for and 76 against in the 300-seat Parliament on Thursday after more than 30 hours of fiery debate over two days. Strong support from the center-left and leftist opposition parties pushed the measure through. (Of the 300 members of the body, a total of 254 people voted. Two of them voted present; the rest abstained.)

Mr. Mitsotakis hailed the vote in a post on social media, describing the new law as “a milestone for human rights.”

Human rights advocates have welcomed the prospect of same-sex marriage for Greece. Maria Gavouneli, the president of the Greek National Commission for Human Rights, an independent public body, called the measure “long overdue.” And Stella Belia, the founder of Rainbow Families, an organization that supports same-sex families, called the legislation “a major victory that we’ve been fighting for for years.”

“It makes life much, much easier for many people, and it protects children that have been living in a state of precariousness,” Ms. Belia said, adding that the new measures will also end the practice of taking children of same-sex couples into the state’s care after the death of a biological parent. Without the new legal protection, she said, “they would lose not one, but both of their parents.”

One of the first to benefit from the new law would be Lio Emmanouilidou, a 43-year-old teacher, who plans to marry her long-term partner in Thessaloniki on March 8, which is International Women’s Day. She said she was excited about the wedding and welcomed the bill as “a step in the right direction and a big victory for the community.”

She lamented, however, that even with its approval, her partner would still face a “long and expensive” adoption process — costing about 3,500 euros, or $3,750 — to become a legal guardian of Ms. Emmanouilidou’s 6-year-old son, whom the partners have raised together as a family. (Under the new bill, both members of a married same-sex couple would automatically be legally recognized as parents of children the pairs give birth to or adopt.)

Ms. Emmanouilidou also said she felt unnerved by the opposition to the measures. But she said that, in her experience, most Greeks accepted same-sex couples and that her school and community treated her family as any other.

“Society is much more ready for this than we think,” she said.

Yet in a country that remains one of Europe’s most socially conservative, where the traditional family model is still predominant and the influential Orthodox Church views homosexuality as an aberration, the measures have met some pointed resistance.

The Holy Synod, the Greek Orthodox Church’s highest authority, argued in a letter to lawmakers this month that the bill “abolishes fatherhood and motherhood, neutralizes the sexes” and creates an environment of confusion for children. Clerics echoed such sentiment in sermons across the country in recent weeks, and some bishops said they would refuse to baptize the children of same-sex couples.

Church groups also joined forces with far-right parties to hold rallies in Athens and other cities to oppose the changes. Last Sunday, hundreds of people staged a demonstration outside Parliament, with some holding banners that read, “There’s only one family, the traditional one.”

Opinion polls conducted in recent weeks depicted a Greek society split over the issues: In most of the surveys, half of respondents expressed support for same-sex marriage, yet most respondents also said they opposed allowing same-sex couples to adopt children.

The bill also fueled dissent across the Greek political spectrum.

In the governing New Democracy party, dozens of lawmakers, including a prominent minister and a former prime minister, argued that the legislation weakened the nuclear family and undermined traditional values. The leader of Greece’s Communist Party, Dimitris Koutsoubas, told Parliament on Thursday that legalizing same-sex marriage would “abolish the unity of motherhood and fatherhood.”

And the issue caused discord within Syriza, the main opposition party: Some lawmakers said the bill did not go far enough, others were loath to back a conservative government’s bill on what they considered a liberal issue and some worried about winning support in rural areas.

Syriza even drafted its own alternative bill, but the party’s leader, Stefanos Kasselakis — who is Greece’s first openly gay party leader and has expressed a desire to adopt children through surrogacy with his partner, whom he married in New York last October — later pressed his fellow lawmakers to back the government’s legislation.

Supporters said the changes were a crucial step toward granting full rights to gay people and their children, and opening up minds in a society where traditional heteronormative attitudes prevail.

“It’s the best we were going to get from a center-right government with that kind of internal opposition and the entire Orthodox Church pressuring you,” Ms. Belia said. “I’ve got to hand it to Mitsotakis for following through.”