Legalising same-sex marriage would "shame Scotland in the eyes of the world", the leader of the country's Catholics has warned.
Cardinal Keith O'Brien said allowing such a change would have "huge implications" for society and would represent a "grotesque subversion of a universally accepted human right".
He also accused ministers of being "disingenuous" and of "staggering arrogance" over suggestions that churches would not be obliged to solemnise gay marriages.
In a strongly-worded article, the cardinal warned that the Catholic Church would do everything it could to "protect" marriage.
He was speaking out after the Scottish Government recently revealed it has begun gathering views on whether same-sex marriage should be made legal.
A 14-week consultation asks if marriage in Scotland should be allowed for gay people through a civil or religious ceremony.
Currently, same-sex couples can enter a civil partnership which carries full legal rights but the ceremony cannot be conducted in a church or other religious premises.
Ministers and officials say they intend to meet key groups to discuss the proposals, which would ensure religious organisations do not have to register same-sex marriages against their will.
A recent Scottish Social Attitudes survey found that more than 60% of people believe same-sex couples should have the right to marry, compared with 19% who disagree.
However, the cardinal warned against changing an ancient tradition.
In an article for the Scottish Mail on Sunday, he wrote: "As an institution, marriage long predates the existence of any state or government. It was not created by government and should not be changed by them, instead recognising the innumerable benefits which marriage brings to society they should act to protect and uphold it not attack or dismantle it."
The leader of the Catholic Church in Scotland added: "The Universal Declaration on Human Rights is crystal clear when it says that marriage is a right which applies to men and women, it goes on to state, that 'the family is the natural and fundamental group unit of society and is entitled to protection by society and the State'. This universal truth is so self evident that it shouldn't need to be repeated."
"If the Scottish Government attempts to demolish a universally recognised human right, it will have forfeited the trust which the nation, including many in the Catholic community, have placed in it, and its intolerance will shame Scotland in the eyes of the world."