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Federal Liberal and Labor MPs have lined up to condemn a same-sex marriage supporter who allegedly headbutted former prime minister Tony Abbott in Hobart.
Mr Abbott was walking toward his waterfront hotel when he was approached by a man on Thursday who said he wanted to shake his hand but then headbutted him.
"A fellow sung out at me -- 'Hey Tony'. I turned around. There was a chap wearing a Vote Yes badge," Mr Abbott told Macquarie radio.
"He says 'I want to shake your hand'. I went over to shake his hand then he headbutted me.
"He wasn't very good at it, I've got to say, but he did make contact.
"The only damage was a very, very slightly swollen lip."
A staffer from Mr Abbott's office tussled with the man before he ran off "swearing his head off".
"It's just a reminder of how ugly this debate is getting," Mr Abbott said.
Mr Abbott lodged a formal complaint with Tasmania Police, which is investigating.
Prime Minister Malcolm Turnbull phoned Mr Abbott to check on his welfare and also discussed related security issues with Australian Federal Police commissioner Andrew Colvin.
Mr Abbott was in Tasmania campaigning for the 'no' vote in the national same-sex marriage survey along with Liberal party colleague Senator Eric Abetz.
Senator Abetz said he had dinner with Mr Abbott later on Thursday and he was in "fine form".
He agreed the incident was out of character for the broader 'yes' campaign but said ugliness on social media showed some would be empowered if the vote was successful.
"This is just a bit of a harbinger of what is likely to occur, and it's not the sort of Australia I want," he told ABC TV.
Cabinet minister Christopher Pyne said no one should be attacked for having a different view on marriage and it must have come as a nasty shock for Mr Abbott.
"It is an un-Australian thing to do and I hope that Tony is okay," he told the Nine Network.
Deputy Liberal leader Julie Bishop echoed that sentiment.
"Violence of any form is never acceptable," she told reporters in New York.
Opposition Leader Bill Shorten described the attack as unacceptable.
"I'm glad Mr Abbott isn't seriously injured and I've rung him to say so," he said on Twitter.
Labor frontbencher Penny Wong said the yes campaign was about hope, respect and unity and the debate should reflect those values.
"Violence is never acceptable," she said.
Australian Marriage Equality campaigner and Independent NSW Parliament MP Alex Greenwich also slammed the attack.
"There is no room for any disrespect either physical or verbal in this national debate," he said.
Tasmania Police confirmed it was investigating the alleged assault on Mr Abbott, 59, which occurred about 4.35pm.
"Police would particularly like to hear from the other man who was allegedly involved in the incident and have urged him to come forward," it said in a statement.
Mr Abbott's injuries were described as "minor".
He's believed to have departed Hobart for Melbourne early on Friday.