Slovenia's citizens are holding a referendum on whether to allow same sex-marriage for the first time in any of the former communist countries of Central and Eastern Europe.

Sunday's vote was forced by conservative groups, backed by the Catholic Church, who seek to overturn a bill that defines marriage as a union between two consenting adults.

Parliament passed the marriage equality amendment in March, but the "Children are At Stake" group has collected 40,000 signatures to challenge the changes before any gay couples were able to marry.

At least 20 percent of Slovenia's 1.7 million voters must reject the bill for it to be overturned. Recent opinion polls suggest voters remain sharply divided over the issue.