Stab victim ignored on Melbourne footpath

An assault victim lay seriously injured near an inner-Melbourne tram stop while people walked past him, police say.

The man aged in his 60s remains in hospital with serious head and back injuries after being assaulted and losing consciousness early on Friday on the William Barak Bridge, near the Melbourne Park tennis precinct.

"(He) had been there for quite some time," Detective Senior Constable Todd Greaney told reporters on Wednesday.

"Several members of public walked past him and police want those people to come forward.

"There was a trail of blood that was approximately 80 metres long from the William Barak Bridge tram stop to the gardens of Birrarung Marr."

The trail also led to several pieces of clothing covered in blood, Const Greaney said.

The man, who was assaulted between 4am and 7am before a passer-by called police, remains so badly injured that detectives haven't been able to speak with him.

"He's extremely lucky he's still alive," Const Greaney said.

Investigators have released CCTV of the victim walking along the footpath before he passed out.

A composite image of a man police would like to speak to about the assault was released last week.