David Cameron is to face a mass petition opposing plans to launch a "hostile strike" on the traditional view of marriage, campaigners said today.
Christian groups backed by former Archbishop of Canterbury Lord Carey said they were sending 175,000 emails out this week inviting people to sign a petition opposing the introduction of same-sex marriage.
The group Coalition for Marriage (C4M) has said the Government has no democratic mandate to introduce the legislation and has betrayed a promise made by the last Labour administration to maintain a distinction between civil partnerships and marriage.
Lord Carey described plans to introduce same-sex marriage as a "hostile strike" and an act of "cultural and theological vandalism" against an institution dating back thousands of years.
"The avowed intention to widen the scope of marriage as we see before us is a hostile strike, which rather than strengthening marriage, will destroy its meaning and diminish its importance drastically," he told a news conference in central London.
"The legal and theological definition of marriage is that of a man and a woman in a lifelong relationship.
"The Government has many difficult duties to perform on behalf of the nation it is elected to serve.
"But it is not in its gift to alter such a fundamental relationship."
He added: "This matter is so serious and so important for our nation - we cannot allow this act of cultural and theological vandalism to happen."
The campaign, which has the backing of the Roman Catholic Church in England and Wales, is opposed to Government plans to legislate for same-sex marriages by 2015.
A public consultation on how to make civil marriage available to same-sex couples is to be launched next month.
The launch of the campaign was supported by Fiona Bruce, MP for Congleton and David Burrowes, MP for Enfield, Southgate.
Mr Burrowes said he had received a "barrage of hostile hate-filled emails and tweets" including a death threat after publicity about his opposition to same sex marriage.
"I have been subjected to hate mail, to accusations of homophobia and to a death threat as well which is extraordinary when one is affirming what the law is as it stands and has done historically," he said.
"We must at the very least be able to have this debate in a reasoned manner that respects freedom and also recognises that if we do redefine marriage it will impact on people's freedoms - on that teacher who is wanting to affirm their religious conscience in teaching traditional marriage.
"If the law is changed then they will be challenged by that."
He added that he was "fully respectful" of civil partnerships.
Mr Burrowes' remarks come after it was revealed earlier this month that police were investigating racist emails sent to the Archbishop of York Dr John Sentamu following his criticism of plans to allow same sex marriage.
Lord Brennan QC, the Catholic peer, told the news conference: "This is a matter of national significance. Many, many times in our public lives, Lord Carey and I have heard the phrase 'let the people speak', this is a classic occasion for letting the people speak.
"We cannot afford to allow social engineering to take place producing the Orwellian results of 'parent one and parent two' instead of mother and father.
"Such changes are for the people to decide on."
Gay rights campaigner Peter Tatchell commenting on the launch, said: "The Coalition for Marriage is intolerant and out of touch.
"Its support for the ban on gay marriage is homophobic and discrimination.
"Coalition members are entitled to believe that same-sex marriages are wrong, but they are not entitled to demand that their opposition to such marriages should be imposed on the rest of society and enforced by law."