* Catholic group wants man-woman marriage in constitution
* Centre-left government, human rights groups oppose demand
* Opinion polls suggest referendum will pass
* EU leaves same-sex laws to every member state
Croats voted in a referendum on Sunday to determine if marriage as a "union of man and woman" should be enshrined in the constitution, a move initiated by Roman Catholic groups but criticised by opponents as discriminatory against gays.
Parliament in the new European Union member state called the referendum after the Catholic "In the Name of the Family" group gathered over 740,000 signatures in support of the vote.
Calls for the referendum grew after the Social Democrat-led government last year introduced sex education in schools, then hinted it would grant a right to same-sex couples to be treated as if they were married, such as over next-of-kin status and inheritance, although without allowing them to formally marry.
Croatia, which joined the European Union in July, is a staunchly Catholic country with some 90 percent of its 4.4 million people saying they belong to the Church, which wholeheartedly supported the referendum.
The EU leaves regulation of same-sex rights to each member state.
Croatian priests urged believers during Sunday Mass services to vote "yes" and "defend our traditional values", and many were likely to heed that call.
"I voted in favour. Since the earliest times people know that marriage is only between man and woman and I hope God will help us make this referendum succeed," an elderly lady who gave her name as Ivanka said after casting her ballot.
In a poll this week, 68 percent of those who have decided to vote said they would support the demand. There is no turnout threshold for the referendum to be valid and its results, expected after 1900 GMT, are binding.
Government figures, including Prime Minister Zoran Milanovic, have said from the start they disagreed with the referendum's demand and would vote against it.
"SAD AND POINTLESS" REFERENDUM
Milanovic, among the first to vote on Sunday morning, called the referendum "sad and pointless".
"The intimate space of a family is not something we should get involved in," he told reporters. He said the government would next week propose a bill on common law partnership to grant wider rights to same-sex unions.
Opponents of the referendum said it was discriminatory and anti-gay. Leaders of the conservative opposition HDZ party have supported the referendum demand.
In Croatia, gay pride marches have become routine but same-sex partners still face many practical challenges.
"Right now, I have no right to inherit my girlfriend's property, we cannot adopt children or get married, I could not visit her if she ended up in an old people's home," said Marta Susak, a 21 year-old student who came to vote holding hands with her girfriend Mima Simic.
"We want to be treated like all other Croatian citizens. With the introduction of marriage between man and woman in the constitution, we are denied that right and are defined as second-class citizens," said Simic, a 37-year old film critic.
The group that demanded the vote says it wants to make sure the definition of marriage is in the constitution so that it cannot be changed by amendments to the family law, which do not require a two-third majority in parliament.