Officers were called to the corner of George and Liverpool Streets in Sydney after a man was assaulted around 2.10am on Monday.
A 25-year-old man was allegedly involved in a fight with six other men he did not know.
The men fled the scene in a blue Holden Commodore.
Paramedics treated the 25-year-old at the scene where he was later taken to hospital with a fractured skill and bleeding brain.
He remains in critical condition.
Police have released images of six men they believe are involved in the incident.
They are all described as being of sub-continental Indian appearance.
The first man is described as 180-185cm tall, of solid build, with facial hair and short black hair. He wore a blue puffer jacket and a blue and red “Ellesse” brand shirt and jeans.
The second man is around 180-185cm tall, he is of solid build, with facial hair and short black hair. He wore a light-colored hoodie and dark sweatpants.
The third man is 170-175cm tall, medium build, with well-trimmed facial hair and short black hair. He has a tattoo on the back of his neck.
He was wearing jeans, a light colored shirt with a light colored ‘Puma’ brand down jacket.
The fourth man is about 170-175cm tall, medium build, with facial hair and short black hair combed to one side. He wore sweatpants, black sandals, and a light-colored zippered sweater.
The fifth man is about 170-175cm tall, medium build, with facial hair and short black hair. He wore light colored sweatpants, black flip flops and a hoodie.
The sixth man is about 170-175cm tall, short in stature, and has short black hair. He wore a dark jacket and light jeans.
Anyone who knows the men or witnessed the incident is asked to contact Crime Stoppers on 1800 333 000.