An American tourist has required emergency surgery to stop bleeding on his brain after he was bashed in a terrifying attack near a tram stop in Melbourne's north.

Key points: The offender bashed the victim twice, taunting and pointing at him when he passed out

The offender bashed the victim twice, taunting and pointing at him when he passed out The victim was helped by a passer-by after he regained consciousness 25 minutes later and walked towards a main road

The victim was helped by a passer-by after he regained consciousness 25 minutes later and walked towards a main road He had been in Australia for six weeks, and was due to fly home to the US on Saturday

Police released security camera footage showing the 32-year-old victim, who was beaten unconscious twice as he walked away from a tram near the intersection of Bell Street and Plenty Road in Preston, about 11:20pm on Wednesday.

The assault was thought to be a random attack, with police describing it as "terrifying" and the worst assault they have seen.

"He strikes him to the point he's unconscious, hovers over him and appears to taunt him," Detective Senior Constable Matthew Coleiro said.

"Once he regains consciousness he again continues the assault until he's unconscious again, hovers over him again, then leaves him in the middle of the road unconscious."

The 32-year-old eventually regained consciousness and walked towards a main road for help, where he was found by a passer-by almost half an hour after the attack happened.

"It appears to be unmotivated at this point, the victim is seen quite clearly stepping off a tram walking on his own, this unknown male runs up to him and we don't know why at this point," Senior Constable Coleiro said.

"The vision is very confronting, we're only releasing parts of it."

The victim, a travelling musician visiting Melbourne, has required two surgeries since the attack to stop the bleeding on his brain, as well as for a broken jaw.

He was due to fly home on Saturday after spending six weeks in Australia.

The American tourist suffered bleeding on the brain and has needed multiple surgeries. ( Supplied: Victoria Police )

"His family have been notified overseas, he's got no other real support network here in this country," Senior Constable Coleiro said.

"He wants to get back home to find some comfort and support.

"The victim is unfortunately suffering some serious memory loss as a result of the assault, so he can barely recall the incident occurring."

Police said they were relying on the CCTV and witnesses coming forward to identify the offender.

The assailant is described as slim, aged in his early 30s, balding with dark hair and a dark beard.

He was wearing a t-shirt with a white motif along the front, knee-length cargo shorts and a big backpack.

"Hopefully we can find out who this person is and he'll get some reassurance and some comfort in that," Senior Constable Coleiro said.

Anyone with information, or who witnessed the attack, should phone CrimeStoppers on 1800 333 000.