Scotland's Roman Catholic priests on Sunday condemned plans by the devolved government in Edinburgh to legalise gay marriage.
The letter by Scotland's Catholic bishops, which was being read in each of the Church's 500 Scottish parishes, urges followers to fight efforts to "redefine" marriage.
"Marriage is a unique lifelong union of a man and a woman," the letter argues.
Scotland's devolved government announced plans in July to become the first part of Britain to legalise same-sex marriage, but it stressed that churches would not be forced to host gay weddings.
Civil partnerships for same-sex couples were introduced in Britain in 2005, giving them similar rights to married heterosexual couples -- but the partnerships cannot legally be referred to as marriages.
The letter expresses the Roman Catholic Church's "deep disappointment" at the Scottish plans, and announces the launch of a National Commission for Marriage and the Family to co-ordinate a campaign against gay marriage.
"The Church's teaching on marriage is unequivocal," said Cardinal Keith O'Brien, the leader of Scotland's Catholics.
"It is uniquely, the union of a man and a woman and it is wrong that governments, politicians or parliaments should seek to alter or destroy that reality."
"While we pray that our elected leaders will sustain rather than subvert marriage, we promise to continue to do everything we can to convince them that redefining marriage would be wrong for society."
The Scottish government has pledged to bring forward a bill on gay marriage later this year, and hopes the first ceremonies could take place at the start of 2015.
The proposal comes after 65 percent of the nearly 80,000 people who took part in a Scottish government consultation supported same-sex marriage.