HELSINKI (Reuters) - The Finnish Parliament on Friday narrowly approved a citizen's initiative to legalise same-sex marriage.

Gay couples in Finland have been able to enter into registered partnerships since 2002, but until now the country was the only one in the Nordic region to not allow same-sex marriage. Finland is now the 12th European state to do so.

In the vote, 105 members of parliament supported the legal amendment while 92 opposed it.

The measure will end the distinction in Finland between same-sex unions and heterosexual marriages and give such couples equal rights to adopt children and share a surname.

"Finland should strive to become a society where discrimination does not exist, human rights are respected and two adults can marry regardless of their sexual orientation," centre-right Prime Minister Alexander Stubb said in an open letter before the vote.

Most opponents argued that all children should have the right to a father and mother. "This is a question of the future of our children and the whole society, and such changes should not be made without thorough evaluation of their impact," Mika Niikko of the nationalist Finns party said before the vote.

(Reporting by Jussi Rosendahl and Anna Ercanbrack; Editing by Mark Heinrich)