ZAGREB (Reuters) - Croatians will vote on December 1 whether to define marriage in the constitution as "a union of man and woman", a referendum sponsored by a Roman Catholic civic group and criticized by opponents as discriminatory and anti-gay.
Parliament, controlled by a liberal center-left coalition, formally approved the referendum on Friday after a Catholic group called "In the Name of the Family" gathered more than 740,000 signatures in support of the vote.
Croatia, which joined the European Union in July, is a staunchly Catholic country and more than 90 percent of its 4.4 million people declare themselves to be in the church. The EU leaves regulation of same-sex rights to every member state.
Zagreb's Social Democrat-led government last year introduced sex education in schools despite strong protests from the Catholic church and the conservative camp.
It then upped the ante by hinting it planned to grant conjugal rights to same-sex couples, though not the right to formally marry, after which demands for a referendum grew.
"We want to be sure that something as fundamental as marriage and family, and all the ensuing rights, cannot be changed by simply changing the family law," Zeljka Markic, head of In the Name of the Family, told a Croatian Catholic weekly.
She said Croatia should avoid mass street protests like the ones that happened in France at the start of this year when President Francois Hollande legalized same-sex marriage and adoption rights.
The two biggest political parties are split on the issue.
Prime Minister Zoran Milanovic said this week he was against the referendum and would probably vote "No".
"In this initiative I see a lack of tolerance towards a (sexual) minority group. Marriage is not threatened by same-sex groups but by the modern way of life, the race to earn money," Milanovic told state television HRT.
Tomislav Karamarko, the head of the opposition conservative HDZ party, praised the referendum and said he would vote "yes".
The public ombudsman, Lora Vidovic, said the referendum violated the constitution, which bans any discrimination. "Such a definition of marriage would put common law and same-sex unions at a disadvantage," she said in a statement.
There is no turnout threshold for the referendum results to be valid.
Reporting by Zoran Radosavljevic; Editing by Mark Heinrich