In 2011, while on probation after serving prison terms for graffiti, the US duo fled their homeland for India, and have spray-painted trains and walls in more than 30 countries in Africa, Europe and Asia since.

They released books and films, held exhibitions, and regaled their cult-like following online with tales from their so-called "Probation Vacation".

But Mr Harper's five-year celebrity world tour came to an end last week when Luke*, a single father from Fitzroy, wrestled him to the ground in a headlock for allegedly tagging shopfronts on Brunswick Street.

"When I had him pinned down and he heard the siren in the distance, he begged me to let him go," Luke said.

"I said 'you punched me in the face dickhead, I'll let you go when the cops get here.'"

Mr Harper, who tags as Ether, is behind bars, remanded until May 31.

But his partner in life and crime, Ms Bremner, or Utah, remains on the lam, almost certainly in Melbourne. If caught, she is likely to face charges and then extradition to the US with Mr Harper, where the pair will be prosecuted for breaching probation.

Another man wanted for the Brunswick Street graffiti is a prolific Australian graffitist know as Nokier, who has painted with the duo across the world.

It is believed police do not know his identity, but have released an image of the man, taken by Luke shortly before he performed the citizen's arrest of Mr Harper.

Nokier is shown in one of the films released by the pair, Tunisia Takeover, and is also believed to have travelled with them to India. It is unclear if this friendship drew the notorious pair to Melbourne.

Luke said he saw Mr Harper and Nokier tagging and putting stickers on shops along Brunswick Street, near Rose Street, about 2.30pm on May 4.

He asked them to stop, and started filming with his phone, when they lashed out, assaulting him and trying to steal his phone, which was in a case with cash and bank cards.

"In the tussle somehow [we] pocket dialed my sister, who then heard me getting punched in the face by both men.

"She freaked out and called 000 but didn't know where I was. She's totally traumatised."

Luke does not want to be identified, fearing he may be subject to a revenge attack for performing a citizen's arrest on a world-famous graffitist.

Mr Harper, 31, was allegedly found with a knife, and is charged with attempted robbery, recklessly cause injury, unlawful assault, possess controlled weapon, and four counts of criminal damage relating to offences allegedly committed on Brunswick Street.

He is also charged with defacing trains in Bayswater, Williamstown, Kensington and Pakenham.

Mr Harper and Ms Bremner have received coverage in the New York Times and New York Post for their exploits, but online interviews since they fled the US best articulate their ethos.

"Even being in possession of basic art materials like paint or markers would have been in violation of our probation," the couple told website The Hundreds.

"We decided that we just couldn't live like that, putting our lives on hold for five years. We knew we wanted to go somewhere new, somewhere we hadn't been before and somewhere where there were lots of walls, trains and metros to paint."

Mr Harper, from Chicago, ran with a well-known US-crew, Made U Look. Ms Bremner, 34, who also paints as Dani, attended the Fashion Institute of Technology in her native New York.

They were arrested after returning from Europe in 2008 and charged with tagging trains over the past three years.

"When we got out, we knew that we wouldn't stop painting," they said at the start of last year.

"Who knows, maybe you'll see our names rolling up to a train station in your city sometime soon."

Anyone who may be able to identify Nokier or who saw him at the time of the incident is urged to contact Crime Stoppers on 1800 333 000 or crimestoppersvic.com.au.