David Connolly has been sworn in as Northern Territory's administrator - the equivalent of a state governor - at Parliament House in Darwin as about 100 people gathered outside with placards.
Mr Connolly issued an apology this week after facing mounting pressure from critics over his previous "offensive" social media posts denigrating Indigenous people.
NT Speaker Robyn Lambley had earlier denied permission for the rally, saying there was "insufficient capacity to accommodate a demonstration" outside parliament at the same time as the ceremony.
Protesters defied the ban on Friday, gathering outside parliament and shouting their opposition to Mr Connolly's appointment.
A man was reportedly arrested outside Parliament House before protesters surrounded a police van shouting "shame" and "let him go" as it tried to leave.
The protest did not disrupt the ceremony, with Mr Connolly sworn in as the territory's 24th administrator following an opening address featuring Governor-General Sam Mostyn and NT Chief Minister Lia Finocchiaro.
The chief minister said the veteran cattleman had a longstanding connection to the NT and extensive industry experience, making him "well suited" to the role.
"His Honour has spent nearly two decades working in the Northern Territory, building strong relationships across regional and remote communities," she said.
Since Mr Connolly's late 2025 nomination by NT's Country Liberal Party government there have been calls for it to be quashed, put on hold or reviewed due to his questionable X posts.
They included attacks on what he called Indigenous privilege, an apparent mocking of Welcome to Country ceremonies and a re-post describing Mr Albanese as an "arsehole".
Mr Connolly issued an apology late on Wednesday, saying he deeply regretted his posts that were deleted but recently re-emerged in the media.
He said in a statement he was genuinely sorry and did not mean to cause offence with the posts that were "no reflection of my personal core values".
Northern Territory MP Yingiya Guyula said it would take more than a last-minute apology for people to believe Mr Connolly was genuinely sorry.
"He has a clear record that everyone can see of being offensive," he said in a statement to AAP.
"He was the wrong choice for the position.Â
"He should turn the role down. This would show he can see the damage his views have caused."
Mr Guyula was among dozens of politicians, advocates and community groups who signed a letter to Prime Minister Anthony Albanese this week requesting he "urgently rescind" the vice-regal appointment to restore public confidence.
The NT MP also issued a personal plea to Mr Albanese to withdraw his support for Mr Connolly.
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