Former Archbishop of Canterbury Lord Carey appears to have compared anti-gay marriage campaigners to persecuted Jews in Nazi Germany.
Addressing a Coalition for Marriage rally on the fringes of the Tory conference, he criticised supporters of reform - including broadcaster Stephen Fry - for calling opponents bigots or extremists.
Calling for a "sensible debate", he suggested verbal abuse of Jewish people marked the beginning of the totalitarian Nazi state.
"Remember that the Jews in Nazi Germany, what started it against them was when they were called names, that was the first stage towards that totalitarian state," he said.
Deputy Prime Minister Nick Clegg was criticised last month after early drafts of a speech he was due to make branded opponents of gay marriage "bigots". The extracts were later withdrawn.
Lord Carey also warned that gay marriage would pave the way for traditional Mormon-style polygamous relationships, pointing to an application in Sao Paulo, Brazil, for a man to enter into a civil union with two women.
"That is getting into a Mormon-style relationship," he said after the event. "It's a slippery slope."
Lord Carey said he was "perplexed" that a Conservative government could push proposals that would strike at the heart of society and may have "dramatic consequences".
He told the rally that for "time immemorial", marriage had been between and man and a woman and gay relationships were not the same.
He said: "Same-sex relationships are not the same as heterosexual relationships and should not be put on the same level. Why does it feel to us that our cultural homeland and identity is being plundered?"