Romania is holding two days of voting on a proposed change to its constitution that would define marriage as "a union between a man and a woman” instead of "a union between spouses."
Same-sex marriage is already prohibited under Romanian law.
Critics say a change in the wording of the constitution would make it just about impossible for gays and lesbians to marry in the future.
The country's LGBT community says the referendum will do nothing more than make people feel like second-class citizens and will fuel homophobia even further.
The ruling Social Democrats are responsible for bringing the measure, which is supported by the country’s Orthodox Church, to a vote on Saturday and Sunday.
The referendum has alarmed Brussels and the European Union Commission's deputy chief has reminded Bucharest of its human rights commitments.
Frans Timmermans said recently at a debate on Romanian reforms: "I don't want family values to be transformed into arguments that encourage the darkest demons and hatred against sexual minorities."
Civil rights groups have urged voters to boycott the referendum. "In a democracy, the rights of minorities are not put to a vote. That's the difference between the Middle Ages and the 21st century," said the Center for Legal Resources, a non-profit NGO.
Thirty percent of the country's registered voters must participate in the referendum for the vote to be valid.