Roman Catholic priests have read out a letter in each of the Church's parishes in Scotland criticising the Scottish government for plans to introduce gay marriage.
Last week the leader of the Church in Scotland, Cardinal Keith O'Brien, broke off personal discussions on the issue with the First Minister, Alex Salmond.
The Church has declared 26 August as National Marriage Sunday.
The Scottish government says it is right to introduce same-sex marriage.
But it has stressed no clergy would be forced to carry out the ceremonies.
The letter criticises the Scottish government for trying to change what Cardinal O'Brien considers to be the only legitimate definition of marriage - the lifelong union of a man and a woman.
The letter also announces the launch of a National Commission for Marriage and the Family to co-ordinate a campaign against gay marriage.
It says: "We reaffirm before you all the common wisdom of humanity and the revealed faith of the Church that marriage is a unique life-long union of a man and a woman."
And it asks the Church's congregation to "pray for our elected leaders, invoking the Holy Spirit on them, that they may be moved to safeguard marriage as it has always been understood, for the good of Scotland and of our society".
In March, Cardinal O'Brien described gay marriage as a "grotesque subversion of a universally accepted human right".
The Scottish government has pledged to bring forward a bill on the issue later this year, and has indicated the earliest ceremonies could take place by the start of 2015.
Same-sex couples in Scotland can already enter into civil partnerships. The Scottish government has promised to protect religious freedoms, and says any changes would not see churches forced to take part in same-sex ceremonies.
A Scottish government spokesperson said: "We intend to proceed with plans to allow same sex marriage and religious ceremonies for civil partnerships because we believe it is the right thing to do.
"We are equally committed to protecting religious freedom and freedom of expression, and ensuring that religious celebrants opposed to same sex marriage do not have to solemnise same sex ceremonies."
The Equality Network, which is campaigning in support of same-sex marriage in Scotland, said politicians should stand firm over the plans.
Tom French, the charity's policy co-ordinator, said: "It is increasingly clear that the Church has an anti-gay agenda that it wants to impose on the rest of society.
"We urge the Scottish government to stand firm on plans to introduce equal marriage and not give in to demands that would discriminate against LGBT people."
Other Churches in Scotland have also expressed concern about the same-sex marraige plans. The Church of Scotland has said it is concerned the Scottish government is "rushing ahead on something that affects all the people of Scotland without adequate debate and reflection".
And the Free Church of Scotland described the decision to introduce gay marriage as a "truly sad day for Scotland" and urged the Scottish government to reconsider its plans.
The UK government's consultation on allowing civil marriages for gay and lesbian couples in England and Wales closed in June.
The coalition is analysing the results before publishing its formal response, but Prime Minster David Cameron has promised legislation before the next election.
There are currently no plans for similar legislation in Northern Ireland.