Sydney - A man wearing a "yes" badge headbutted former Australian prime minister Tony Abbott while he was campaigning against legalising gay marriage, the ex-leader said on Friday, fuelling fears a heated postal vote on the issue could turn violent.Up to 15 million Australians taking part in the voluntary postal ballot can choose "yes" or "no" on whether gay marriage should be legalised, with the results to be released in mid-November.Abbott, an outspoken critic of such unions, said he was in Hobart in south Tasmania on Thursday when a man approached wanting to shake his hand, but then headbutted him instead."I held out my hand. He grabbed my hand and turned it into a headbutt. Now, he was, of course, wearing a 'yes' badge," Abbott told reporters in Hobart on Friday."I worry about the brave new world of same-sex marriage if this is how some of the people who are most enthusiastically working for it are behaving with such bullying and intimidatory fashion."The 59-year-old suffered minor injuries to his lip but "didn't require medical assistance", Tasmania Police Commander Tony Cerritelli told reporters.Abbott, long renowned for his conservative views, was the prime minister from 2013 to 2015, when he was deposed by current leader Malcolm Turnbull - a moderate who supports marriage equality.Turnbull on Friday condemned the alleged headbutt, adding that "one incident is one too many"."However, it's important to remember that overwhelmingly Australians are engaging in this debate respectfully and harmoniously," he said.'No room for disrespect'Leading "yes" campaigner Alex Greenwich, an independent New South Wales state MP, said there was "no room for any disrespect either physical or verbal in this national debate".Concern the vote could unleash abuse and homophobic slurs saw parliament pass election-style safeguards restricting campaign material that might be misleading and deceptive during the ballot period.The incident occurred as pop star Elton John, who is performing a series of shows in northern Queensland which was devastated by a category-four cyclone earlier this year, called on Australians to embrace marriage equality."I love Australia. I love its spirit, its lack of pretence, its passion. I hope it can embrace the honesty and courage that seeks gay marriage as an expression not of desire but of love," he wrote on Facebook late Thursday.The postal survey is non-binding, but Turnbull has vowed to hold a vote in parliament if the majority of Australians choose "yes".