The Catholic Church has written a letter to be read out in every parish in Scotland criticising the Scottish Government's plans to legalise gay marriage.
The Church has declared August 26 as National Marriage Sunday to highlight its opposition to what it describes as a "grotesque subversion of a universally accepted human right".
The Scottish Government has insisted it will go ahead with its plans to make same-sex marriages lawful, but the clergy will not be compelled to conduct them.
Cardinal Keith O'Brien, the leader of the Church in Scotland, last week broke off discussions on the issue with First Minister Alex Salmond.
The Equality Network, which is campaigning in support of same-sex marriage, called for politicians to stand firm against the Church's "anti-gay agenda".
In a strongly worded message, the letter will highlight the church's "deep disappointment that the Scottish Government has decided to redefine marriage and legislate for same sex marriage."
It will also announce the launch of a National Commission for Marriage and the Family to co-ordinate a campaign against gay marriage.
Cardinal O'Brien, who has described gay marriage as a "grotesque subversion of a universally accepted human right", said: "The Church's teaching on marriage is unequivocal, 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."
He added: "With this letter we will announce the creation of a National Commission for Marriage and the Family, a body which will be charged with promoting the true nature of marriage, it will develop an online presence and produce materials and organise events which will help Catholic families to support and sustain marriage."
Tom French, policy co-ordinator for The Equality Network, 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."
"In particular, the Equality Network would be deeply concerned at any attempt to promote an anti-gay agenda in schools. School should be a welcoming environment for all young people, regardless of their sexual orientation or their family situation."
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