CANBERRA, Australia >> Australia’s prime minister today urged restraint in the nation’s same-sex marriage debate after a former prime minister who gave the people a direct say in the divisive issue said he was head-butted by a gay rights advocate while walking in a city street.

Former Prime Minister Tony Abbott said he sustained a swollen lip in the attack in the Tasmania state capital Hobart on Thursday as he walked to his hotel after attending an anti-gay marriage luncheon.

The altercation is one of several allegations of violence and vote-rigging that have marred an ongoing postal ballot on whether Australia should lift its prohibition on gay marriage. Australia and Ireland are the only countries to put the issue to the public to decide.

Abbott remains a government lawmaker and is a vocal advocate for the “no” vote. The two-month voting process began last week.

He reported to police that a man pretended to want to shake hands before clashing heads. The assailant fled after a brief scuffle with a lone Abbott staffer on the scene.

“It’s a shock to have a fellow Australian seeking to shake your hand turn a handshake into an assault,” Abbott told reporters on Friday.

“It’s sad that this debate has come to that and my plea to everyone in the remaining weeks of this debate is to keep it courteous, keep it respectful, but above all else respect the values, the institutions which have shaped us since the beginning of our journey and which I think should continue to shape us,” he added.

Tasmania Police Commander Tony Cerritelli said detectives were looking for a 40-year-old suspect of medium build, with spiky, fair hair and nose and ear piercings. He was wearing a “vote yes” badge and black leather jacket.

Prime Minister Malcom Turnbull, a gay marriage supporter who replaced Abbott two years ago, said he had contacted the Australian Federal Police commissioner to urge that the assailant be charged with assault.

“This is a disgraceful incident and I condemn this assault on Tony and any violence coming into our political life is absolutely to be condemned,” Turnbull told 3AW.

“Everyone should exercise calm and restraint, and people should recognize that they are not helping their cause by engaging in violent language, abusive language, let alone physically violent conduct … as Tony suffered from last night,” Turnbull added.

Advocates on both sides of the debate have been punished for taking public stances in the campaign.

The Australian Football League headquarters in Melbourne was evacuated by a bomb hoax on Thursday, a day after it removed a “yes” logo from its front door. The logo had been displayed for only a day and football officials had been inundated with complaints from the public for taking a position in the heated debate.

A Presbyterian church minister announced last week that he would refuse to marry a couple in a Ballarat church after the 26-year-old bride-to-be revealed on social media her support for same-sex marriage.

An 18-year-old woman was fired from a Canberra children’s party business last weekend because she posted on social media: “It’s OK to vote ‘no’,” a slogan her employer described as “hate speech.”

Abbott was prime minister in 2015 when he committed his conservative government to holding a compulsory vote by all adult Australians to decide whether gay marriage should be legal.

He was replaced weeks later by Turnbull, who opposed the public vote but agreed to maintain Abbott’s policy in a deal with the powerbrokers who ousted Abbott in an internal leadership ballot.

The Senate refused to fund a compulsory vote, so the government is proceeding with a voluntary postal ballot which critics say is unlikely to accurately reflect public opinion.

Lawmakers would still have to pass a law to allow gay marriage and several have said they would not allow it regardless of public opinion.

Most gay rights advocates had argued against a public vote for fear that it would lead to a bitter public debate. They want lawmakers to decide the issue without consultation with the public.

Successive opinion polls in recent years show that most Australians support marriage equality.

Kevin Rudd, a center-left Labor Party prime minister whom Abbott defeated in elections in 2013, blamed the postal ballot for an assault on his godson Sean Foster, 19, as he campaigned for marriage equality in Brisbane city last week.

“So many warnings to Turnbull about what the postal vote cld unleash. Now my godson Sean has been punched standing up for #MarriageEquality,” Rudd tweeted with a photograph showing Foster’s bloodied forehead.

Police have charged a 48-year-old man with assaulting Foster.