On 1 December voters will decide whether to approve changes to Croatia's constitution, to define marriage as a union between a man and a woman.
More than two-thirds of the members of parliament in Croatia backed the plan.
A petition in favour of the plan, drawn up by a Catholic group, received more than 700,000 signatures. Opponents say it is discriminatory and anti-gay.
On Friday the parliament of the majority-Catholic country voted to hold a referendum asking the question: "Do you agree that marriage is matrimony between a man and a woman?"