The Scottish Government announced in July that it proposes bringing in legislation to allow same-sex couples to wed.
Ministers declared that it is the “right thing to do” and the move received backing from equality campaigners and cross-party support at Holyrood.
But the controversial plan is vigorously opposed in other quarters, such as the Catholic Church and many other religious leaders.
The petition was set up by Scotland For Marriage, an umbrella group formed to fight changes to marriage laws. It said the number of people who have signed its petition has now passed the 30,000 mark, as it vowed to redouble efforts to block the legislation.
A spokesman said: “Our campaign is going from strength to strength as the implications of redefining marriage on everyday lives becomes more widely understood.”
The Equality Network, a charity that campaigns for same-sex marriage, said a clear majority of people in Scotland supports equal rights for lesbian, gay, bisexual and transgender people.
Policy co-ordinator Tom French said: “All the opinion polls show that a two-thirds majority of Scots support equal marriage rights for same-sex couples.”