The Archbishop of Glasgow, Mario Conti, has written to all Scottish parishes to urge them to oppose plans to give gay couples the right to marry.
He said that churches would “actively campaign” to block marriage rights for gay people and warned that the move would create “larger divisions” in society.
The archbishop also claimed that allowing gay marriage would lead to “gross discrimination” against Christians.
The government announced a consultation on gay marriage last month and said it was “minded” towards marriage equality.
Although government ministers have stressed that churches will be free to choose whether to hold gay weddings, religious leaders claim that they may be forced to in future.
Catholics are being urged to sign a declaration against gay marriage.
Writing to parishes, Archbishop Conti said that the government does not have a mandate to “reconstruct society on ideological grounds”.
He added: “The Catholic Church, for one, will not accept it, and indeed will actively campaign against it.
“It will act to create larger divisions in society and could lead in the future, as we have seen in some of the legislation to date, to gross allegations of discrimination.
“There will be other consequences in law, and social policies stemming from it which need to be taken into account, for example housing provision, social security entitlements and the legitimate expectations of families for support in having and bringing up children on whom the future of society depends, and for which society should make provision.”
Tim Hopkins, of gay rights group The Equality Network, said that the church was launching its “biggest campaign in years”.
He said: “Only the Catholic bishops can explain why their top political priority is not poverty, abuse or war, but is stopping loving couples from marrying.”
The leader of the Scottish Catholic Church, Cardinal Keith O’Brien, said last month that gay parenting “means that our society deliberately chooses to deprive a child of either a mother or a father”.
He added: “All children deserve to begin life with a mother and father, the evidence in favour of the stability and well being which this provides is overwhelming and unequivocal. It cannot be provided by a same sex couple however well intentioned they may be.”