Police believe that a 21-year-old man was attacked by a small number of people in the early hours of Wednesday morning.
Part of the pavement on Upper Parliament has been cordoned off along with Trinity Walk and Kings Walk forcing businesses in the area to close.
The assault took place in the early hours of Wednesday.
The man is still in a “serious condition” at the Queens Medical Centre.
A police spokesperson said that no arrests have been made.
Witnesses came to the victim’s aid and an investigation is under way as officers try to work out who is an attacker and who was trying to help.
Anyone who has information should telephone Nottinghamshire Police on 101, quoting incident number 63 of April 15 – alternatively, they can call Crimestoppers on 0800 555 111.
Police are reviewing CCTV footage from that time as part of an on-going investigation.