Croatians on Sunday are likely to approve a referendum which seeks to limit marriage to a heterosexual union.

If a majority of voters agree that “marriage is matrimony between a man and a woman,” then Croatia's constitution will be amended to define marriage as solely the union of a man and a woman.

Polling shows that nearly 60 percent of voters favor the amendment.

A petition in support of amending the constitution has received more than 700,000 signatures and was organized by the Catholic group In the Name of the Family. Catholics are being urged to support the measure. Nearly 90 percent of Croatians are Roman Catholics.

Croatia's Cardinal Josip Bozanic told followers: “Marriage is the only union enabling procreation. This is the key difference between a marriage and other unions.”

Hundreds opposed to the measure rallied Saturday in Zagreb, Croatia's capital. They argue that passage would be a major setback for the country, which joined the EU in July.

“I will vote against because I think that the referendum is not a festival of democracy, but a festival of oppression against a minority, which fights for its rights and which does not have its rights,” a university student told the AP.

Croatian President Ivo Josipovic has also urged a “no” vote.

“The question is: Do we need this kind of a referendum? I think that determining marriage between a man add a woman does not belong in the constitution. A nation is judged by its attitude toward minorities,” Josipovic said.

Human rights groups opposed to the referendum as an infringement of basic human rights said they planned on filing an appeal with the country's constitutional court.